Serializer

Doesn’t appear to handle undefined very well
```
**plugins.serialize.toJSON({a : null, b: undefined})
//yields: {“javaClass”:“org.mozilla.javascript.NativeObject”,“_b”:{“javaClass”:“org.mozilla.javascript.Undefined”},“_a”:null}

plugins.serialize.fromJSON(plugins.serialize.toJSON({a : null, b: undefined}))
//Throws exception: org.jabsorb.serializer.UnmarshallException: could not instantiate bean of type org.mozilla.javascript.Undefined**

```

will be fixed in Servoy 5.2.10