Ensures all auth handlers correctly parse all auth headers.
* bzrlib/tests/test_http.py: (TestAuthHeader): Test for all known auth schemes.
* bzrlib/transport/http/_urllib2_wrappers.py: (AbstractAuthHandler._parse_auth_header): All handlers should be able to parse any header, they may as well share the implementation. (NegotiateAuthHandler.auth_match, BasicAuthHandler.auth_match, DigestAuthHandler.auth_match): JFDI.