Fix for issue #206

Fix for issue #https://github.com/atk4/atk4/issues/206.

Should change return $this to return $this->render() because we need to return rendered expression here and not query database directly.

Important:
1) this needs more testing, but I guess it should be fine - works for me.
2) I guess all other DSQL "return $this;" lines should be checked twice for same issue.