Merge pull request SDWebImage#1049 from hoppenichu/fix_donwloader_2

Make sure callbacksForURL won't be modified

Author: Olivier Poitrey

SDWebImage/SDWebImageDownloader.m

@@ -138,7 +138,7 @@ - (void)setOperationClass:(Class)operationClass {
                                                              if (!sself) return;
                                                              __block NSArray *callbacksForURL;
                                                              dispatch_sync(sself.barrierQueue, ^{
-                                                                 callbacksForURL = sself.URLCallbacks[url];
+                                                                 callbacksForURL = [sself.URLCallbacks[url] copy];
                                                              });
                                                              for (NSDictionary *callbacks in callbacksForURL) {
                                                                  SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
@@ -150,7 +150,7 @@ - (void)setOperationClass:(Class)operationClass {
                                                             if (!sself) return;
                                                             __block NSArray *callbacksForURL;
                                                             dispatch_barrier_sync(sself.barrierQueue, ^{
-                                                                callbacksForURL = sself.URLCallbacks[url];
+                                                                callbacksForURL = [sself.URLCallbacks[url] copy];
                                                                 if (finished) {
                                                                     [sself.URLCallbacks removeObjectForKey:url];
                                                                 }