update lib/once.js to match clientside definition

lib/once.js

@@ -1,8 +1,7 @@
-module.exports = function (f) {
+module.exports = function (f, g) {
-    var called;
     return function () {
-        if (called) { return; }
+        if (!f) { return; }
-        called = true;
         f.apply(this, Array.prototype.slice.call(arguments));
+        f = g;
     };
 };