def __call__(environ, status, headers):
    """ args -> True | False

        o 'environ' is the WSGI environment.

        o 'status' is the HTTP status as returned by the downstream
          WSGI application.

        o 'headers' are the headers returned by the downstream WSGI
          application.

        This interface is responsible for returning True if
        a challenge needs to be presented to the user, False otherwise.
        """
