Fix broken UIButton KVC code

Exception was raised in [`UIButton+AFNetworking.m:107`](https://github.com/AFNetworking/AFNetworking/blob/2490d50d352d838fb3dfc2be2dfa5dc3047e9427/UIKit%2BAFNetworking/UIButton%2BAFNetworking.m#L107) (link current as of this writing).