AFImageRequestOperation tweaks

This pull request fixes a couple of issues I was having when using AFImageRequestOperation.

* One of the servers my app tried to connect to was returning JPEG images with a content type of `image/jpg`. While technically incorrect, this caused the operation to fail, which isn't desirable behaviour. Unfortunately the server isn't one I control, otherwise I'd have made the change on the server side.

* While attempting to subclass `AFImageRequestOperation` to include the invalid type, I noticed that `imageRequestOperationWithRequest:imageProcessingBlock:success:failure:` always returned AFImageRequestOperation instances, rather than instances of the current class (like the other request operation classes).