Given a message, publish to this topic.

def _send(self, message):
        """
        Given a message, publish to this topic.
        """
        message['command'] = 'zappa.asynchronous.route_sns_task'
        payload = json.dumps(message).encode('utf-8')
        if len(payload) > LAMBDA_ASYNC_PAYLOAD_LIMIT: # pragma: no cover
            raise AsyncException("Payload too large for SNS")
        self.response = self.client.publish(
                                TargetArn=self.arn,
                                Message=payload
                            )
        self.sent = self.response.get('MessageId')