Compare NSURLRequest rather than NSURL

If you inadvertently load a nil URL in UIImageView+AFNetworking, the failure block of the image request operation is bypassed because [nil isEqual:nil] == NO. By comparing the URL request instead of the URL, the `isEqual:` test passes and the error is properly forwarded to the failure block parameter.
Merge pull request #772 from 0xced/fix-uiimageview-nil-url

Compare NSURLRequest rather than NSURL