Test protocol version in http responses.

* bzrlib/transport/http/_urllib2_wrappers.py:
(AbstractHTTPHandler.do_open): Catch httplib.UnknownProtocol. Add some
comments.

* bzrlib/transport/http/__init__.py:
(BadProtocolRequestHandler): New class testing protocol validity.

* bzrlib/tests/test_http.py:
(TestWallServer, TestBadStatusServer.create_transport_readonly_server,
TestInvalidStatusServer): Use a readonly server.
(TestBadProtocolServer, TestBadProtocolServer_urllib): New classes
testing protocol version validity.

* bzrlib/tests/__init__.py:
(TestCaseWithMemoryTransport.create_transport_readonly_server): New
hook allowing daughter classes to create specialized servers.