Modify fs.open to use accept a callback without a mode

lib/fs.js

@@ -140,8 +140,11 @@ fs.closeSync = function (fd) {
   return binding.close(fd);
 };
 
-fs.open = function (path, flags, mode, callback) {
+fs.open = function (path, flags, mode_, callback) {
-  if (mode === undefined) { mode = 0666; }
+  var mode = (typeof(mode_) == 'number' ? mode_ : 0666);
+  var callback_ = arguments[arguments.length - 1];
+  var callback = (typeof(callback_) == 'function' ? callback_ : null);
+
   binding.open(path, stringToFlags(flags), mode, callback || noop);
 };