util: improve setting default options

Object.assign is currently very slow. Using Object.keys is much
faster in v8 6.3.

Author: BridgeAR

lib/util.js

@@ -317,7 +317,9 @@ Object.defineProperty(inspect, 'defaultOptions', {
     if (options === null || typeof options !== 'object') {
       throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'options', 'Object');
     }
-    Object.assign(inspectDefaultOptions, options);
+    const keys = Object.keys(options);
+    for (var i = 0; i < keys.length; i++)
+      inspectDefaultOptions[keys[i]] = options[keys[i]];
     return inspectDefaultOptions;
   }
 });