Completion block not being called

[tonyxiao]

https://github.com/path/FastImageCache/blob/master/FastImageCache/FICImageCache.m#L226

If delegate gives back nil for entity, then completion block in asynchronouslyRetrieveImageForEntity:withFormatName:completionBlock will not be called, breaking the API contract and documentation, which says

 @note You can always rely on the completion block being called. If an error occurs for any reason, the `image` parameter of the completion block will be `nil`. See

Fixed by commit 213a3ff.