-canProcessRequest isn't accurate

Hi Mattt,

While I was working with Tumblr's API, I realized that AFHTTPClient doesn't use the earlier registered operation classes. Tumblr's API doesn't have a suffix (e.g. .json) appended thus let -canProcessRequest fail. 
IMO, I'd suggest to rename the method so it implies a probability to be able to process the request since it's not always right. It's your library and you know the best how to handle this issue. 

I hope you can fix this issue or merge my fork. 
As always, great work. I'm totally in love with it and use it in every project I can.