Convert message data to :class:`Message` if needed, and
        merge with the default message.

        :param message: Message to merge with the default message.
        :rtype: :class:`Message`

def _cast_message(self, message=None):
        '''Convert message data to :class:`Message` if needed, and
        merge with the default message.

        :param message: Message to merge with the default message.
        :rtype: :class:`Message`
        '''
        if message is None:
            message = {}
        if isinstance(message, Mapping):
            message = Message.load_message(message)
        return message.load_from(self.message, verify=True)