Fixed race condition betwen operation cancelation and loading finish

When using UICollectionView that uses SDWebImage to download content and scroll it fast there are much operation cancelations and when you stop scrolling part of images would not be loaded.

It is because of  race condition between connectionDidFinishLoading and cancel methods in SDWebImageDownloaderOperation that leads to NSURLErrorTimedOut instead of image loads.

The problem is 2 calls of CFRunLoopStop() by one from each of the methods. First cancels current thread and next some thread that starts download of other image.