SDWebImageDownloader: Fixed threading issues in which completion selectors where sometimes not executed

Affected “-(id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionWithFinishedBlock)completedBlock” method
Merge remote-tracking branch 'obviousengineering/3.7.1-oe'