fix: fix this refrence bug

Author: luozhang002

lib/cluster.js

@@ -215,7 +215,7 @@ module.exports = function (OssClient) {
   };
 
   proto._createCallback = function _createCallback(ctx, gen, cb) {
-    return function () {
+    return () => {
       cb = cb || this._error;
       gen.call(ctx).then(() => {
         cb();