util: fix isPrimitive check

Previous check failed for the edge case Object.create(null). This uses
the current v8 code for the check.

lib/util.js

@@ -506,7 +506,12 @@ function isFunction(arg) {
 exports.isFunction = isFunction;
 
 function isPrimitive(arg) {
-  return !(arg instanceof Object);
+  return arg === null ||
+         typeof arg === 'boolean' ||
+         typeof arg === 'number' ||
+         typeof arg === 'string' ||
+         typeof arg === 'symbol' ||  // ES6 symbol
+         typeof arg === 'undefined';
 }
 exports.isPrimitive = isPrimitive;