FOLDABLE TENT FRAME BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tent frame, and more particularly to a foldable 5 tent frame. 2. Description of the Prior Art Early days, traditional tents need several persons to complete the assembly of a tent. This assembly is inconvenient, so the manufacturers are devoted to improve this problem. Chinese Patent No. 023238607.9 discloses a foldable tent frame as 10 shown in Fig. 1, which comprises four upper rods 1', two middle rod 2', two lower rods 3', a connection rod 4', a middle connection sleeve 5', a lower connection sleeve 6', a rotatory buckle 7', and a plurality of sleeves 8'. The four upper rods 1' are symmetrically connected to the connection rod 4'. The two middle rods 2' are symmetrically connected to the middle connection sleeve 5'. The two lower rods 3' 15 are symmetrically connected to the lower connection sleeve 6'. The middle connection sleeve 5' and the lower connection sleeve 6' are cooperated with the connection rod 4'. The rotatory buckle 7' is disposed on the connection rod 4'. The sleeves 8' are disposed on the joints of the upper rods 1'. The two middle rods 2' are hinged to the middle portions of the upper rods 1'. When extending, the tent frame is 20 fixed through the rotatory buckle 7'. There are many procedures to assemble the tent frame. Accordingly, the inventor of the present invention has devoted himself based on his many years of practical experiences to solve these problems. SUMMARY OF THE INVENTION The primary object of the present invention is to provide a tent frame which is 25 simple for operation and has a better rigidity, or at least to provide the public with a useful choice. In order to achieve the aforesaid object, the foldable tent frame of the present invention comprises an upper connection head, a guide rod, a plurality of poles, a plurality of first support rods, a middle connection head, a plurality of second support 30 rods, and a lower connection head. Inner ends of the plurality of poles are pivotally connected to the upper connection head. The guide rod has an upper end which is 1 fixedly connected to a lower end of the upper connection head and a lower end which extends downward. The plurality of first support rods each have an inner end which is pivotally connected to the middle connection head and an outer end which is pivotally connected to the inner end of the respective pole. The middle connection 5 head has a middle guide hole for insertion of the guide rod. The middle connection head is moved up and down along the guide rod. The plurality of second support rods each have an inner end which is pivotally connected to the lower connection head and an outer end which is pivotally connected to a middle portion of the respective pole. The lower connection head has a middle guide hole for insertion of 10 the guide rod. The lower connection head is moved up and down along the guide rod. Preferably, the foldable tent frame further comprises inner hinge members. Each inner hinge member is in an L shape, and has an inner end which is pivotally connected to the upper connection head and an outer end which is fixedly connected 15 to the inner end of the respective pole. Each inner hinge member has a protrusion for connection of the outer end of the respective first support rod. Preferably, the foldable tent frame further comprises middle hinge members. Each the middle hinge member is fitted on the middle portion of the respective pole and has a protrusion for connection of the outer end of the respective second support 20 rod. Preferably, the number of the poles is four, the number of the first support rods is two, and the number of the second support rods is four. Accordingly, the inner ends of the first support rods are pivotally connected to the middle connection head, and the outer ends of the first support rods are pivotally 25 connected the inner ends of the poles. Through this configuration, the first support rods provide a support between the middle connection head and the poles when the tent is unfolded, so that the tent won't fold up automatically. The present invention doesn't need the rotatory buckle of the conventional tent frame, which is simple in configuration and operation. It is very convenient to unfold and fold the tent. The 30 present invention has four second support rods to provide a better rigidity. The present invention can be folded and unfolded by one person, which can be used conveniently. 2 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a conventional tent frame; Fig. 2 is an exploded view according to a preferred embodiment of the present 5 invention; Fig. 3 is a cross-sectional view according to the preferred embodiment of the present invention; Fig. 3A is a cross-sectional view of the middle connection head according to the preferred embodiment of the present invention; 10 Fig. 3B is a cross-sectional view of the lower connection head according to the preferred embodiment of the present invention; and Figs. 4A through Fig. 4C are schematic views of the preferred embodiment of the present invention in a fully unfolded state, a semi-unfolded state, a semi-folded state, and a fully folded state. 15 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. As shown in Fig. 2 and Fig. 3, the foldable tent frame of the present invention comprises an upper connection head 1, a guide rod 2, four poles 3, two first support 20 rods 4, inner hinge members 5, a middle connection head 6, four second support rods 7, middle hinge members 8, and a lower connection head 9. Inner ends of the four poles 3 are pivotally connected to the upper connection head 1. The guide rod 2 has an upper end which is fixedly connected to a lower end of the upper connection head 1 and a lower end which extends downward. 25 Each inner hinge member 5 is in an L shape. Each inner hinge member 5 has an inner end which is pivotally connected to the upper connection head 1 and an outer end which is fixedly connected to the inner end of the respective pole 3. Each inner hinge member 5 has a protrusion 51 thereon. Each first support rod 4 has an outer end which is pivotally connected to the protrusion 51 and in inner end which is 3 pivotally connected to the middle connection head 6. The outer ends of the two first support rods 4 are pivotally connected to the inner ends of two symmetrical poles 3 of the four poles 3. As shown in Fig. 3A, the middle connection head 6 has a middle guide hole 61 for insertion of the guide rod 2. The middle connection head 6 can be 5 moved up and down along the guide rod 2. When the middle connection head 6 is moved up and down along the guide rod 2, the first support rods 4 connected with the middle connection head 6 brings the poles 3 to fold or to unfold. Each middle hinge member 8 is fitted on a middle portion of the respective pole 3, and has a protrusion 81 thereon. Each second support rod 7 has an outer end which 10 is pivotally connected to the protrusion 81, so that the outer ends of the four second support rods 7 are pivotally connected to the middle portions of the four poles 3. Each second support rod 7 has an inner end which is pivotally connected to the lower connection head 9. As shown in Fig. 3B, the lower connection head 9 has a middle guide hole 91 for insertion of the guide rod 2. The lower connection head 9 can be 15 moved up and down along the guide rod 2. Referring to Fig. 4A, the middle connection head 6 and the lower connecting head 9 are fitted on the guide rod 2 and moved up along the guide rod 2, so the middle connection head 6 and the lower connecting head 9 are moved to approach the upper connection head 1, such that the tent frame is fully unfolded. 20 Referring to Fig. 4B, the middle connection head 6 is movably fitted on the guide rod 2 and moved downward and the lower connection head 9 is detached from the guide rod 2, so that the middle and lower connection heads 6, 9 are away from the upper connection head 1. The tent frame is gradually folded. As shown in Fig. 4C, the middle connection head 6 is movably fitted on the guide 25 rod 2 and further moved downward. The lower connection head 9 is fully detached from the guide rod 2, so that the middle and lower connection heads 6, 9 are away from the upper connection head 1. The tent frame is fully folded. Although particular embodiments of the present invention have been described in detail for purposes of illustration, various modifications and enhancements may be 30 made without departing from the spirit and scope of the present invention. Accordingly, the present invention is not to be limited except as by the appended claims. 4