undefined becomes null when serializing

```
interface IEditorModel {
timeDimension?: TimeDimension;
}
const defaultEditorModel: IEditorModel = {
timeDimension: undefined,
};

export class EditorModel extends Record(defaultEditorModel, "EditorModel") implements IEditorModel {
public constructor(values?: Partial) {
super(values);
}
}
const em = new EditorModel();
expect(em.timeDimension).toBeUndefined();
console.log(recordTransit.toJSON(em));
```
This code serializes `timeDimension` to `null` even though the first `expect` passes. Any ideas why this is happening?