Set the message payload (and update header)

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

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

        :param value: New payload value
        :type value: unicode
        :rtype: None
        """
        self._payload = value
        self._header.payload_length = len(self._payload)