Handle servers proposing several authentication schemes.

* bzrlib/transport/http/_urllib2_wrappers.py:
(AbstractAuthHandler.auth_required): Several schemes can be
proposed by the server, try to match each one in turn.
(BasicAuthHandler.auth_match): Delete dead code.

* bzrlib/tests/test_http.py:
(load_tests): Separate proxy and http authentication tests as they
require different server setups.
(TestAuth.create_transport_readonly_server): Simplified by using
parameter provided by load_tests.
(TestAuth.test_changing_nonce): Adapt to new parametrization.
(TestProxyAuth.create_transport_readonly_server): Deleted.

* bzrlib/tests/http_utils.py:
(DigestAndBasicAuthRequestHandler, HTTPBasicAndDigestAuthServer,
ProxyBasicAndDigestAuthServer): Add a test server proposing both
basic and digest auth schemes but accepting only digest as valid.