util: add isPrimitive check

lib/util.js

@@ -505,6 +505,11 @@ function isFunction(arg) {
 }
 exports.isFunction = isFunction;
 
+function isPrimitive(arg) {
+  return !(arg instanceof Object);
+}
+exports.isPrimitive = isPrimitive;
+
 function isBuffer(arg) {
   return arg instanceof Buffer;
 }