Store the session data in redis

        :param method callback: The callback method to invoke when done

def save(self):
        """Store the session data in redis

        :param method callback: The callback method to invoke when done

        """
        result = yield gen.Task(RedisSession._redis_client.set,
                                self._key, self.dumps())
        LOGGER.debug('Saved session %s (%r)', self.id, result)
        raise gen.Return(result)