OUTDOOR UNIT OF REFRIGERATION APPARATUS TECHNICAL FIELD The present invention relates to an outdoor unit of a refrigeration apparatus. BACKGROUND ART 5 Outdoor units of a refrigeration apparatus have been reported to have a polypropylene cap or a cage-shaped electrical-corrosion-preventing body provided to the lower end of a header manifold; e.g., as in patent document 1 (Japanese Laid-open Patent Application No. 2011 145029). However, in cases where water traveling along the header manifold enters the cap or 10 accumulates in a floor frame (a drain pan) in such a configuration, there is a risk of the water infiltrating from a reticulated portion of the cage, contacting the header manifold, and inducing electrical corrosion of the header manifold. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these 15 matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application. Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, 20 or group of elements, integers or steps. SUMMARY According to the present disclosure, there is provided an outdoor unit of a refrigeration apparatus which includes a heat exchanger, a floor frame, and a cap. The heat exchanger is made of aluminum or an aluminum alloy. The heat exchanger has a plurality of flat pipes 25 arranged in a row facing a side face, a header manifold to which each of the flat pipes is connected, and a plurality of fins joined to the flat pipes. In the heat exchanger, exchange of heat occurs between a fluid flowing inside the flat pipes and flowing outside the flat pipes. The heat exchanger is mounted on the floor frame. The header manifold is arranged substantially vertically. The plurality of flat pipes is arranged substantially horizontally in 30 rows towards. The cap covers the lower end part of the header manifold from below. The cap has a first drainage structure allowing water that has entered via a gap between the header manifold and the cap to drain out. In an embodiment disclosed herein, by having the cap placed on the lower end of the header manifold and an opening part for draining water provided in the cap, it is possible for 1 dew from the heat exchanger that has entered via a gap between the header manifold and the cap to drain out, and heat exchanger corrosion and refrigerant leakage to be prevented. Examples of the drainage structure include an opening, a cutout, and a gutter. Dew from the heat exchanger includes condensation water and rainwater. 5 The material of the cap is preferably rubber in terms of enabling safe mounting of the header manifold as well as providing waterproofness and vibration resistance; examples including natural rubber, chloroprene rubber, nitrile rubber, butyl rubber, ethylene-propylene rubber, styrene-butadiene rubber, silicone rubber, fluorine rubber, AFLAS, hydrogenated nitrile rubber, and urethane rubber. The rubber is preferably a hard rubber (hardness: 50). 10 The cap may be configured from a bottom face part and a side face part, and the first drainage structure may be provided in the side face part. According to an embodiment disclosed herein, having the drainage structure provided in the side face of the cap allows a larger drainage structure to be used, dew from the heat exchanger that has entered via a gap between the header manifold and the cap to be more 15 efficiently drained to the outside, and heat exchanger corrosion and refrigerant leakage to be more effectively prevented. The first drainage structure may be an opening. The bottom face of the opening may be provided below the lower end of the header manifold. Furthermore, the bottom face of the opening may be set to incline downward from the inside toward the outside. 20 According to an embodiment disclosed herein, having the opening provided in the side face of the cap, the bottom face of the opening below the lower end of the header manifold, and the bottom face of the opening set at an incline allows condensation that has entered into the cap to be more efficiently routed outside the cap and guided to the floor frame, and heat exchanger corrosion and refrigerant leakage to be more effectively prevented. 25 A projection may be provided on the surface facing opposite the bottom face part of the floor frame. According to an embodiment disclosed herein, the lower end of the header manifold is raised by the cap, which allows infiltration of water into the header manifold to be prevented even when condensation accumulates in the floor frame, and heat exchanger 30 corrosion and refrigerant leakage to be further prevented. Furthermore, condensation that has accumulated in the floor frame can be prevented from flowing into the cap and coming into contact with the header manifold. A second drainage structure may be provided in the projection. 2 According to an embodiment disclosed herein, condensation that has entered into the cap can be guided from the drainage structure of the lower part of the cap to the floor frame, which allows heat exchanger corrosion and refrigerant leakage to be prevented. Examples of the drainage structure include an opening, a cutout, and a gutter or the 5 like. The cap may be designed so that there is no gap through which water can enter between the cap and the header manifold. According to an embodiment disclosed herein, the lower end of the header manifold does not come into contact with water even when condensation accumulates in the floor 10 frame, which allows heat exchanger corrosion and refrigerant leakage to be more effectively prevented. In an outdoor unit of a refrigeration apparatus according to the present disclosure, having the cap placed on the lower end of the header manifold and the opening part for draining water provided in the cap makes it possible for dew from the heat exchanger that has 15 entered via a gap between the header manifold and the cap to drain out, and heat exchanger corrosion and refrigerant leakage to be prevented. In an outdoor unit of a refrigeration apparatus according to the present disclosure, having the drainage structure provided in the side face of the cap allows a larger drainage structure to be used, dew from the heat exchanger that has entered via a gap between the 20 header manifold and the cap to be more efficiently drained out, and heat exchanger corrosion and refrigerant leakage to be further prevented. In an outdoor unit of a refrigeration apparatus according to the present disclosure, having the opening provided in the side face of the cap, the bottom face of the opening below the lower end of the header manifold, and the bottom face of the opening set at an incline 25 allows condensation that has entered into the cap to be more efficiently routed outside the cap and guided to the floor frame, and heat exchanger corrosion and refrigerant leakage to be further prevented. In an outdoor unit of a refrigeration apparatus according to the present disclosure, because the lower end of the header manifold is raised by the cap, infiltration of water into the 30 header manifold can be prevented even when condensation accumulates in the floor frame, and heat exchanger corrosion and refrigerant leakage can be further prevented. Furthermore, the condensation accumulated in the floor frame can be prevented from flowing into the cap and coming into contact with the header manifold. 3 In an outdoor unit of a refrigeration apparatus according to the present disclosure, because condensation that has entered into the cap can be guided from the drainage structure of the lower part of the cap to the floor frame, heat exchanger corrosion and refrigerant leakage can be prevented. 5 In an outdoor unit of a refrigeration apparatus according to the present disclosure, because the lower end of the header manifold does not come into contact with water even when condensation accumulates in the floor frame, heat exchanger corrosion and refrigerant leakage can be further prevented. BRIEF DESCRIPTION OF THE DRAWINGS 10 FIG. 1 is a general perspective view of an outdoor unit of a refrigeration apparatus according to one embodiment of the present invention. FIG. 2 is a general perspective view of an outdoor unit of a refrigeration apparatus according to one embodiment of the present invention in a condition having removed a ceiling plate, a left side plate, a right front plate, and a right rear plate. 15 FIG. 3 is a general perspective view of an outdoor heat exchanger. FIG. 4 is a front view of a cap according to one embodiment of the present invention. FIG. 5 is a plan view of a cap according to one embodiment of the present invention. FIG. 6 is a sectional view along VI-VI in FIG. 4. FIG. 7 is a sectional view along VII-VII in FIG. 5 showing a header manifold mounted 20 on the cap. FIG. 8 is a front view of a cap in Modified example B. FIG. 9 is a plan view of a cap in Modified example C. FIG. 10 is a front view of a cap in Modified example C. DESCRIPTION OF EMBODIMENTS 25 An embodiment of the present invention is described below while referring to the drawings. The outdoor unit of a refrigeration apparatus of the present embodiment is used as an outdoor unit of an air-conditioning apparatus. (1) General configuration of the outdoor unit 30 The outdoor unit of an air-conditioning apparatus according to one embodiment of 4 the present invention is illustrated in FIG. 1. FIG. 1 is a perspective view illustrating the outdoor unit 20 of an air-conditioning apparatus. The outdoor unit 20 is placed outside a space to be air-conditioned (indoors) where air conditioning is to be performed, and an interior of a roughly rectangular box-form casing 50 is divided into a ventilation 5 compartment and a machine compartment by a partitioning plate (not illustrated) extending vertically. The outdoor unit 20 is connected via refrigerant-communicating piping (not illustrated) to an indoor unit (not illustrated) disposed inside the space to be air conditioned. The outdoor unit 20 mainly comprises a roughly box-form casing 50, an outdoor fan (not illustrated), refrigerant circuit-configuring parts (not illustrated) including a heat 10 exchanger, a compressor, valves, pipes, and the like, to configure a refrigerant circuit, and an electrical unit (not illustrated) for performing operation and control. A blow-out opening positioned at the center and to the left of a front face 51 is formed on the casing 50, and air sent out by the outdoor fan is blown out forward from the blow-out opening. 15 The casing 50 has a ceiling plate 57, a right rear plate 56, and a right front plate 55, and additionally has a left side plate 54 and a floor frame 8. "60" indicates a fan grill attached on the outside of the casing 50. FIG. 2 is an overall perspective view of an outdoor unit of a refrigeration apparatus according to one embodiment of the present invention in a condition having removed a top plate, a left 20 side plate, a right front plate, and a right rear plate. A heat exchanger 25 is mounted on a floor frame 8, and an outdoor fan 35 is disposed in front of the heat exchanger 25. The interior is divided by a partitioning plate 58, and as viewed facing the front, the left side is a ventilation compartment and the right side is a machine compartment. The outdoor heat exchanger 25 allows refrigerant flowing inside to be condensed or 25 evaporated by heat exchange with outside air. The outdoor heat exchanger 25 is made entirely of aluminum or aluminum alloy. The outdoor heat exchanger 25 is disposed inside the casing 50 with a proper space from the casing 50, or a resin member, or the like, is disposed between the outdoor heat exchanger 25 and the casing 50, so as not to be in direct contact with the casing 50 (see FIG. 1). 30 The outdoor heat exchange 25, as illustrated in FIG. 2, extends along the back face of the casing 50 toward the left from near an end part of the partitioning plate 58, changes in direction at near a left rear corner part of the casing 50, and extends toward the front along the left side plate 54 (see FIG. 1). FIG. 3 is a general perspective view of an outdoor heat exchanger 25. As illustrated in FIG. 5 3, the outdoor heat exchanger 25 has a flat pipe 2, fins 4, and header manifolds 61 and 62. The fins 4 are omitted in the illustration in FIG. 3. Some of the reference numerals of the flat pipes 2 also are omitted. The header manifolds 61, 62 are connected to the both end of the flat pipes 2 which are arranged in multiple rows towards up and down. The 5 header manifolds 61, 62 have functions of supporting the flat pipes 2, guiding refrigerant to an inner channel in the flat pipes 2 (not illustrated), and letting refrigerant coming out of the inner channel collect. Each of a lower end 6a, 6a of header manifolds 61, 62 is covered from below by a cap 14 to be described later (see FIG. 7). (2) Cap 10 FIGS. 4 through 6 illustrate a front view, a plan view, and a sectional view of a cap 14. FIG. 7 illustrates a sectional view showing a header manifold 6 mounted on the cap 14. The cap 14 is made of a rubber material, and has a bottom face part 15 and a side face part 16. The bottom face part 15 is of a cylindrical configuration. The side face part 16 of the cylindrical configuration stands vertically upright from an edge portion on the 15 circumference of the circle of the bottom face part 15 viewed from above, whereby a hole 19 into which the lower end part 6a of the header manifold 6 is inserted is formed. A lower end 6b of the header manifold 6 is inserted so as to contact an upper face 15b of the bottom face part 15, and the heat exchanger 25 is mounted on the cap 14. The cap 14 is formed from hard rubber (hardness: 50) so that the cap 14 will not come free even if the heat exchanger 25 20 mounted thereon vibrates. The cap 14 has a drainage opening 17 in the side face part 16. Condensation from the heat exchanger 25 that has traveled along the header manifold 6 and entered the gap between the header manifold 6 and the cap 14 is drained out from the drainage opening 17. A bottom face 17a of the drainage opening 17 is provided below the upper face 15b of the 25 bottom face part 15. Therefore, condensation from the heat exchanger 25 that has traveled along the header manifold 6 and entered the gap between the header manifold 6 and the cap 14 can be more effectively drained. Furthermore, the bottom face 17a is set at a downward incline from the inside toward the outside, and drainage of condensation is further promoted. A boss 15a is provided to a surface of the bottom face part 15 facing the floor frame 8. 30 Accordingly, even when water accumulates in the floor frame 8, there is minimal likelihood that the lower face part 6a of the header manifold 6 will be immersed in the water. (3) Features of the outdoor unit (3-1) The outdoor unit 20 of the air-conditioning apparatus 1 according to the present embodiment comprises a heat exchanger 25, a floor frame 8, and a cap 14. The heat 6 exchanger 25 is made of aluminum or aluminum alloy. The heat exchanger 25 has a plurality of flat pipes 2 arranged facing a side face, a header manifold 6 to which each of the flat pipes 2 are connected, and a plurality of fins 4 joined to the flat pipes 2. In the heat exchanger 25, exchange of heat occurs between a fluid flowing inside the flat pipes 2 and air 5 flowing outside the flat pipes 2. The heat exchanger 25 is mounted on the floor frame 8. The cap 14 covers the lower end part 6a of the header manifold 6 from below. The cap 14 has a drainage opening 17 through which water that has entered via a gap between the header manifold 6 and the cap 14 drains out. By having the cap placed on the lower end of the header manifold and an opening part 10 for draining water provided to the cap, it is possible for condensation from the heat exchanger that has entered via a gap between the header manifold and the cap to drain out, and heat exchanger corrosion and refrigerant leakage to be prevented. (3-2) In the outdoor unit 20 of the air-conditioning apparatus 1 according to the present embodiment, the cap 14 is configured from a bottom face part 15 and a side face part 15 16, and the drainage opening 17 is provided to the side face 16. According to this embodiment, having the drainage opening provided to the side face of the cap allows a larger drainage opening to be used, condensation from the heat exchanger that has entered via a gap between the header manifold and the cap to be more efficiently drained out, and heat exchanger corrosion and refrigerant leakage to be more effectively 20 prevented. (3-3) In the outdoor unit 20 of the air-conditioning apparatus 1 according to the present embodiment, the bottom face 17a of the drainage opening 17 is provided below the lower end 6b of the header manifold 6. Furthermore, the bottom face 17a is set at a downward incline from the inside toward the outside. 25 According to this embodiment, having the opening provided to the side face of the cap, the bottom face of the opening below the lower end of the header manifold, and the bottom face of the opening set at an incline allows condensation that has entered into the cap to be more efficiently routed outside the cap and guided to the floor frame, and heat exchanger corrosion and refrigerant leakage to be more effectively prevented. 30 (3-4) In the outdoor unit 20 of the air-conditioning apparatus 1 according to the present embodiment, a boss 15a is provided to a surface of the bottom face part 15 facing the floor frame 8. According to this embodiment, the lower end of the header manifold is raised by the cap, which allows the header manifold to be prevented from being immersed in water even 7 when condensation accumulates in the floor frame, and heat exchanger corrosion and refrigerant leakage to be further prevented. Furthermore, condensation that has accumulated in the floor frame can be prevented from flowing into the cap and coming into contact with the header manifold. 5 (3-5) In the outdoor unit 20 of the air-conditioning apparatus 1 according to the present embodiment, the cap 14 is designed to leave substantially no gap through which water can enter between the cap 14 and the header manifold 6. According to this embodiment, the lower end of the header manifold does not come into contact with water even when condensation accumulates in the floor frame, which allows 10 heat exchanger corrosion and refrigerant leakage to be more effectively prevented. (4) Modified examples A modified example of the present embodiment is presented below. A plurality of modified examples may be appropriately combined. (4-1) Modified example A 15 Although the outdoor unit 20 illustrated in the above embodiment is used in an air conditioning apparatus 1, the outdoor unit is not limited to this and may be used in another refrigeration apparatus. (4-2) Modified example B In the outdoor unit 20 indicated by the embodiments, a drainage opening 18 (see FIG. 20 8) may be provided to the boss 15a. (4-3) Modified example C In the cap 141 of the outdoor unit 20 indicated by the embodiments, a drainage opening 18 may be provided to the bottom face part 15 (see FIGS. 9 and 10). In FIG. 10, "71" indicates a pressure-sensitive adhesive, and "72" indicates release paper. In the 25 resulting structure, water more readily drains from the lower part. INDUSTRIAL APPLICABILITY According to the present invention as above, susceptibility to an effect of metal corrosion can be suppressed, and this is useful for an outdoor unit of a refrigeration apparatus. 30 REFERENCE SIGNS LIST 2 Flat pipe 4 Fin 6, 61, 62 Header manifold 8 Floor frame 8 14 Cap 17 Drainage opening 20 Outdoor unit 25 Outdoor heat exchanger (heat exchanger) 5 35 Outdoor fan 50 Casing 51 Front plate 54 Left side plate 55 Right front plate 10 56 Right rear plate 57 Ceiling plate 58 Partitioning plate 60 Fan grill CITATION LIST 15 PATENT LITERATURE Patent document 1: Japanese Laid-open Patent Application No. 2011-145029 9