rework

tween.cm

@@ -41,7 +41,7 @@ function Tween(obj) {
 Tween.prototype.to = function(props, duration, start_time) {
   for (var key in props) {
     var value = props[key]
-    if (isa(value, object)) {
+    if (is_object(value)) {
       // Handle nested objects by flattening them
       for (var subkey in value) {
         var flatKey = key + '.' + subkey