Minimal fix for 497274.

* bzrlib/transport/http/response.py:
(handle_response): Handle 405, may not be the cleanest solution
but the tests pass.

* bzrlib/transport/http/__init__.py:
(SmartClientHTTPMedium.send_http_smart_request): EErk, untested
code is broken code :-/

* bzrlib/tests/test_http.py:
(OnlyGETHEADRequestHandler.parse_request): A dedicated request
handler emulating a restricted server.
(TestGETHEADServer): Test the behaviour of the http clients
against a server that can't reply to POST.