url: using util._extend for improving profermace

`Object.assign` is much slower than `util._extend` according to
the refs. This change is to convert the `Object.assign` to use
`util._extend` in url module for improving profermance.

Refs: nodejs/CTC#62

lib/internal/url.js

@@ -170,7 +170,7 @@ class URLSearchParams {
       return ctx.stylize('[Object]', 'special');
 
     var separator = ', ';
-    var innerOpts = Object.assign({}, ctx);
+    var innerOpts = util._extend({}, ctx);
     if (recurseTimes !== null) {
       innerOpts.depth = recurseTimes - 1;
     }
@@ -373,7 +373,7 @@ Object.defineProperties(URL.prototype, {
     value: function format(options) {
       if (options && typeof options !== 'object')
         throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'options', 'object');
-      options = Object.assign({
+      options = util._extend({
         fragment: true,
         unicode: false,
         search: true,
@@ -1244,7 +1244,7 @@ defineIDLClass(URLSearchParamsIteratorPrototype, 'URLSearchParamsIterator', {
     if (typeof recurseTimes === 'number' && recurseTimes < 0)
       return ctx.stylize('[Object]', 'special');
 
-    const innerOpts = Object.assign({}, ctx);
+    const innerOpts = util._extend({}, ctx);
     if (recurseTimes !== null) {
       innerOpts.depth = recurseTimes - 1;
     }