util: use Object.create(null) for dictionary object

Fixes nodejs#3788
`arrayToHash()` needs to use `Object.create(null)` for its dictionary object.

Refs: nodejs#3791
PR-URL: nodejs#3831
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/util.js

@@ -159,9 +159,9 @@ function stylizeNoColor(str, styleType) {
 
 
 function arrayToHash(array) {
-  var hash = {};
+  var hash = Object.create(null);
 
-  array.forEach(function(val, idx) {
+  array.forEach(function(val) {
     hash[val] = true;
   });