_fixReplyMarkup(obj) {
    const replyMarkup = obj.reply_markup;
    if (replyMarkup && typeof replyMarkup !== 'string') {
      obj.reply_markup = stringify(replyMarkup);
    }
  }
Fix 'reply_markup' parameter by making it JSON-serialized, as
required by the Telegram Bot API
@param {Object} obj Object; either 'form' or 'qs'
@private
@see https://core.telegram.org/bots/api#sendmessage