Merge pull request #444 from hsoi/scrolling-lockup

Break a retain cycle

SDWebImage/SDWebImageDownloader.m

@@ -160,6 +160,7 @@ static NSString *const kCompletedCallbackKey = @"completed";
             [wself.lastAddedOperation addDependency:operation];
             wself.lastAddedOperation = operation;
         }
+        operation = nil; // break retain cycle
     }];
 
     return operation;