Fix assign

Author: Genarito

lib/index.js

@@ -103,7 +103,7 @@ class BundleTrackerPlugin {
    * @param {Partial<Contents>} contents
    */
   _writeOutput(_compiler, contents) {
-    Object.assign({}, this.contents, contents, {
+    Object.assign(this.contents, contents, {
       assets: mergeObjectsAndSortKeys(this.contents.assets, contents.assets),
       chunks: mergeObjectsAndSortKeys(this.contents.chunks, contents.chunks),
     });