Using self.error instead of self.JSONError in AFJSONRequestOperation.

I recently encountered an API returning `null` as a json object. Since `null` is not a valid JSON object, `-[AFJSONRequestOperation responseJSON]` will generate a `JSONError`. Since a `null` response should have resulted in a success callback for my special case, I looked for an easy way to suppress this error. So I swizzled `-[AFJSONRequestOperation error]` and discarded the error.

Since `-[AFJSONRequestOperation setCompletionBlockWithSuccess:failure:]` was using `self.JSONError` in one place, I would need to swizzle a private selector for this method to work which I don't feel comfortable with. Since `self.error` is already being passed into the failure handler and `self.error` picks up the `JSONError`, no harm is done by changing `self.JSONError` to `self.error`.