util: speed up common case of formatting string

PR-URL: https://github.com/nodejs/io.js/pull/1749
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/util.js

@@ -13,6 +13,8 @@ exports.format = function(f) {
     return objects.join(' ');
   }
 
+  if (arguments.length === 1) return f;
+
   var i = 1;
   var args = arguments;
   var len = args.length;