add kwargs to authenticate method

This way, when using multiple authentication backends (ie, a custom django auth backend and custom Saml2Backend) and the first custom backend does not authenticate the user, this other backend can process the next authentication request attempt without crashing.

Author: ganiserb

djangosaml2/backends.py

@@ -67,7 +67,7 @@ def get_saml_user_model():
 class Saml2Backend(ModelBackend):
 
     def authenticate(self, session_info=None, attribute_mapping=None,
-                     create_unknown_user=True):
+                     create_unknown_user=True, **kwargs):
         if session_info is None or attribute_mapping is None:
             logger.error('Session info or attribute mapping are None')
             return None