Fix order dependent test in proxy.test_server

TestObjectController.test_POST_backend_headers was being too picky about the
order of backend requests which when pushed through eventlet will not have a
stable order.  This change preserves the expectations and assertions while
removing the dependency on the order of the requests.

Change-Id: I7176ccb9223cd3dfc3c922b8b3d81eb514891d05