Set the message payload (and update header)

        :param value: New payload value
        :type value: str
        :rtype: None

def payload_set(self, value):
        """
        Set the message payload (and update header)

        :param value: New payload value
        :type value: str
        :rtype: None
        """
        payload = saveJSON(value, pretty=False)
        super(APPDataMessage, self).payload_set(payload)