allow encoding of null and undefined values too

core.js

@@ -703,15 +703,21 @@
     }
 
     Yeti.Tools.Serializer.prototype._encodeValue = function(key, value) {
-        if (value !== undefined) {
-            switch (value.constructor) {
-                case Array:
-                    this._encodeArray(key, value);
-                    break;
-                default:
-                    this._encodeString(key, value);
-                    break;
-            }
+        switch (value) {
+            case null:
+            case undefined:
+                this._encodeString(key, value);
+                break;
+            default:
+                switch (value.constructor) {
+                    case Array:
+                        this._encodeArray(key, value);
+                        break;
+                    default:
+                        this._encodeString(key, value);
+                        break;
+                }
+                break;
         }
     }