Modify extract_credentials to return None or a (username, password) tuple.
Modify extract_credentials to return None or a (username, password) tuple.

This makes a URL with no credentials consistent with Session.request's
credentials=None default, simplifying the test in basic_auth instead of making
it also check for None.

Roger/couchdb-python,infinit/couchdb-python,djc/couchdb-python,djc/couchdb-python