_setImage is intended to call the super version of the method,
however with the adoption of the new _locked_ prefix, ASImageNode's
implementation would call back into ASNetworkImageNode's implementation.
This restore the intent of the _setImage method.
* [ASMultiplex/NetworkImageNode] Ensure ASImageProtocols are not called with nil arguments.
* [ASBasic/PINRemoteImageDownloader] Do not check for nil downloadIdentifier needlessly.
* [ASImageCacheProtocol] Make synchronouslyFetchedCachedImageWithURL URL nonnull.
