use lazy serialization when etag is passed

Author: sokra

lib/cache/FileCachePlugin.js

@@ -173,7 +173,7 @@ class FileCachePlugin {
 			(identifier, etag, data) => {
 				const entry = {
 					identifier,
-					data: store === "pack" ? data : () => data,
+					data: etag ? () => data : data,
 					etag,
 					version
 				};