…certain condition

Fix #224

When the Stampable is inheriting from a super class, for example

1. The super class `WithDynamicFields` implements Stampable
```
abstract class WithDynamicFields<D> implements Auditable, Stampable<D> {

}
```

2. The actual class extends the super class `WithDynamicFields`

```
class Article extends WithDynamicFields<Article> {

}
```

Currently under this condition, createdBy and lastUpdatedBy value saved to database is always 'N/A', it doesn't happen when class `Article` implements `Stampable` directly.

This commit fixes the above issue