Updates message with default message paramaters.

        :param message: Postmark message data
        :type message: `dict`
        :rtype: JSON encoded `unicode`

def _get_request_content(self, message=None):
        '''Updates message with default message paramaters.

        :param message: Postmark message data
        :type message: `dict`
        :rtype: JSON encoded `unicode`
        '''
        message = self._cast_message(message=message)
        return message.json()