util: speed up common case of formatting string

PR-URL: nodejs#1749
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Jeremiah Senkpiel <[email protected]>

Author: ChALkeR

lib/util.js

@@ -12,6 +12,8 @@ exports.format = function(f) {
     return objects.join(' ');
   }
 
+  if (arguments.length === 1) return f;
+
   var i = 1;
   var args = arguments;
   var len = args.length;