Unmarshal error for banChatMember method

Docs: https://core.telegram.org/bots/api#banchatmember

> Returns True on success.

```
func (bot *BotAPI) Send(c Chattable) (Message, error) {
	resp, err := bot.Request(c)
	if err != nil {
		return Message{}, err
	}

	var message Message
	err = json.Unmarshal(resp.Result, &message) // resp.Result = true => Unmarshal error and throw error to up

	return message, err
}
```

