Define SDWebImageDownloader convenience method

SDWebImage/SDWebImageDownloader.h

@@ -207,6 +207,26 @@ typedef SDHTTPHeadersDictionary * _Nullable (^SDWebImageDownloaderHeadersFilterB
  */
 - (void)setOperationClass:(nullable Class)operationClass;
 
+/**
+ * Creates a SDWebImageDownloader async downloader instance with a given URL
+ *
+ * The delegate will be informed when the image is finish downloaded or an error has happen.
+ *
+ * @see SDWebImageDownloaderDelegate
+ *
+ * @param url            The URL to the image to download
+ * @param completedBlock A block called once the download is completed.
+ *                       If the download succeeded, the image parameter is set, in case of error,
+ *                       error parameter is set with the error. The last parameter is always YES
+ *                       if SDWebImageDownloaderProgressiveDownload isn't use. With the
+ *                       SDWebImageDownloaderProgressiveDownload option, this block is called
+ *                       repeatedly with the partial image object and the finished argument set to NO
+ *                       before to be called a last time with the full image and finished argument
+ *                       set to YES. In case of error, the finished argument is always YES.
+ */
+- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
+                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;
+
 /**
  * Creates a SDWebImageDownloader async downloader instance with a given URL
  *

SDWebImage/SDWebImageDownloader.m

@@ -275,6 +275,11 @@
     UNLOCK(self.operationsLock);
 }
 
+- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(NSURL *)url
+                                                 completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
+  return [self downloadImageWithURL:url options:0 progress:nil completed:completedBlock];
+}
+
 - (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                    options:(SDWebImageDownloaderOptions)options
                                                   progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock