Check optional state parameter.

def check_application_state(self, request, callback):
        "Check optional state parameter."
        stored = request.session.get(self.session_key, None)
        returned = request.GET.get('state', None)
        check = False
        if stored is not None:
            if returned is not None:
                check = constant_time_compare(stored, returned)
            else:
                logger.error('No state parameter returned by the provider.')
        else:
            logger.error('No state stored in the sesssion.')
        return check