Updates all messages in message with default message
        parameters.

        :param message: A collection of Postmark message data
        :type message: a collection of message `dict`s
        :rtype: JSON encoded `str`

def _get_request_content(self, message=None):
        '''Updates all messages in message with default message
        parameters.

        :param message: A collection of Postmark message data
        :type message: a collection of message `dict`s
        :rtype: JSON encoded `str`
        '''
        if not message:
            raise MessageError('No messages to send.')
        if len(message) > MAX_BATCH_MESSAGES:
            err = 'Maximum {0} messages allowed in batch'
            raise MessageError(err.format(MAX_BATCH_MESSAGES))
        message = [self._cast_message(message=msg) for msg in message]
        message = [msg.data() for msg in message]
        return json.dumps(message, ensure_ascii=True)