'Get credentials for authentication.
:param pika.frame.MethodFrame method_frame: The Connection.Start frame
:rtype: tuple(str, str)'

def _get_credentials(self, method_frame):

    (auth_type, response) = self.params.credentials.response_for(method_frame.method)
    if (not auth_type):
        raise exceptions.AuthenticationError(self.params.credentials.TYPE)
    self.params.credentials.erase_credentials()
    return (auth_type, response)
