Fix wrong realm extraction in http basic authentication (reported by Jelmer).
* bzrlib/transport/http/_urllib2_wrappers.py: (BasicAuthHandler.extract_realm): Factor out realm extraction for tests purpose.
* bzrlib/tests/test_http.py: (TestAuthHeader.parse_header): Accept a specific auth handler. (TestAuthHeader.test_basic_extract_realm): Explicitly test realm extraction.