assertMessage will test the method and target URL of requests, attributes of server requests, as well as the existing testing of a response's status code. All messages' bodies and header fields are tested for equivalence (with a special case for JSON response bodies).
While the test suite passes, this commit yields a broken server: replacing ad hoc request objectss with PSR-7 ones is still required, as is emission of PSR-7 responses. Both will come in subsequent commits, with tests
Diactoros was chosen specifically because it includes facilities for emitting responses, something which is awkward to test. The end of this refactoring should see both the Response and Request classes disappear, and the general REST class fully covered (as well as any speculative additions to AbstractHanlder).
