Fix the getSize method which use the default file manager instead of current file manager

SDWebImage/SDImageCache.m

@@ -588,7 +588,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
         NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
         for (NSString *fileName in fileEnumerator) {
             NSString *filePath = [self.diskCachePath stringByAppendingPathComponent:fileName];
-            NSDictionary<NSString *, id> *attrs = [[NSFileManager defaultManager] attributesOfItemAtPath:filePath error:nil];
+            NSDictionary<NSString *, id> *attrs = [_fileManager attributesOfItemAtPath:filePath error:nil];
             size += [attrs fileSize];
         }
     });