Explicitly set `diskCacheReadingOptions` to its default value 0

@@ -17,6 +17,7 @@ static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
         _shouldDecompressImages = YES;
         _shouldDisableiCloud = YES;
         _shouldCacheImagesInMemory = YES;
+        _diskCacheReadingOptions = 0;
         _maxCacheAge = kDefaultCacheMaxCacheAge;
         _maxCacheSize = 0;
     }