Making sure cache queries always callback in the main thread.

- queryDiskCacheForKey:done: now always invokes the callback in the main thread
- removed the now redundant main thread dispatch in the image manager class

SDWebImage/SDImageCache.m

@@ -189,8 +189,10 @@ static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
             [self.memCache setObject:diskImage forKey:key cost:image.size.height * image.size.width * image.scale];
         }
 
+        dispatch_async(dispatch_get_main_queue(), ^
+        {
             doneBlock(diskImage, SDImageCacheTypeDisk);
-
+        });
     });
 }
 

SDWebImage/SDWebImageManager.m

@@ -85,12 +85,9 @@
         if (operation.isCancelled) return;
 
         if (image)
-        {
-            dispatch_async(dispatch_get_main_queue(), ^
         {
             completedBlock(image, nil, cacheType, YES);
             [self.runningOperations removeObject:operation];
-            });
         }
         else
         {