Forcing optional param to exist

[riccardocescon]

Describe the bug

The stopPoll function reuires chatId, messageId and replyMarkup:

  Future<Poll> stopPoll(
      dynamic chatId, int messageId, InlineKeyboardMarkup replyMarkup) async {
    if (chatId is! String && chatId is! int) {
      return Future.error(TelegramException(
          'Attribute \'chatId\' can only be either type of String or int'));
    }
    var requestUrl = _apiUri('stopPoll');
    var body = <String, dynamic>{
      'chat_id': chatId,
      'message_id': messageId,
      'reply_markup': replyMarkup,
    };
    return Poll.fromJson(await HttpClient.httpPost(requestUrl, body: body));
  }

But the telegram api does not require replyMarkup, it's an optional parameter: https://core.telegram.org/bots/api#stoppoll

Expected behavior

The replyMarkup should be nullable

[HeySreelal]

I've added the fix for this in #262, hoping this would get merged soon. In the meantime, if you're looking for a local fix, feel free to check this commit: 00341c9

Regards!