Fix downloader queue using assign instead of strong under iOS 6

@@ -21,7 +21,7 @@
 @property (assign, nonatomic) long long expectedSize;
 @property (strong, nonatomic) NSMutableData *imageData;
 @property (strong, nonatomic) NSURLConnection *connection;
-@property (assign, nonatomic) dispatch_queue_t queue;
+@property (SDDispatchQueueSetterSementics, nonatomic) dispatch_queue_t queue;
 
 @end