Validating response

I'm using `AFDownloadRequestOperation` on my project, but when it receives 416 HTTP status (Requested Range Not Satisfiable), the success block is being called. So I created this fix: every time responseObject is accessed, it validates the response using the (default) `responseSerializer`, which checks acceptable HTTP status codes.

Adding to that, I removed `+ acceptableStatusCodes`  method that is not called on AFNetworking 2.0.

I hope this helps.