The callbacksForURL: method of SDWebImageDownloader now returns a copied array. This should fix the crash in multithread environment such as:
Collection <__NSArrayM: 0x226d9310> was mutated while being enumerated.
Merge pull request #471 from PinchProject/hotfix/fast_enumeration_fix

The callbacksForURL: method in SDWebImageDownloader should return a copy of the mutable array!