post a chat message

        Arguments:
            channel_id:
                Scan token

def post_chat_message(self, merchant_id, channel_id, message):
        """post a chat message

        Arguments:
            channel_id:
                Scan token
        """

        return self.do_req('POST',
                           self.base_url + '/chat/v1/merchant/%s/channel/%s/message/' % (merchant_id, channel_id),
                           message)