Merge pull request SDWebImage#886 from hsoi/fix-implicit-atomicity-declaration

Fix implicit atomicity declaration of the SDWebImageManager.cacheKeyFilter

Author: bpoplauschi

SDWebImage/SDWebImageManager.h

@@ -166,7 +166,7 @@ SDWebImageManager *manager = [SDWebImageManager sharedManager];
 
  * @endcode
  */
-@property (copy) SDWebImageCacheKeyFilterBlock cacheKeyFilter;
+@property (nonatomic, copy) SDWebImageCacheKeyFilterBlock cacheKeyFilter;
 
 /**
  * Returns global SDWebImageManager instance.