UPDATE_FUNCTION_GET_A_POST

- [x] create design
   - [x] Spec file
   - [x] class diagram
   - [x] flow diagram
   - [x] sequence diagram
- [x] coding
   - [x] get_a_post function in controller 
   - [x] get_a_post function in model
- [x] unit testing
   - [x] test_get_a_post function in controller
   - [x] test_get_a_post function in model