Generates the messaging-type-related part of the message dictionary.

def serialize(self):
        """
        Generates the messaging-type-related part of the message dictionary.
        """

        if self.response is not None:
            return {'messaging_type': 'RESPONSE'}

        if self.update is not None:
            return {'messaging_type': 'UPDATE'}

        if self.tag is not None:
            return {
                'messaging_type': 'MESSAGE_TAG',
                'tag': self.tag.value,
            }

        if self.subscription is not None:
            return {'messaging_type': 'NON_PROMOTIONAL_SUBSCRIPTION'}