docs: 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)