Bug with selected background view in grouped tableview.

If set new corner radius to selected background view it will be little smaller than background view. And colored corners will be visible, when select as show on screenshot.
![cell_selected_background_view](https://f.cloud.github.com/assets/3592457/694664/3f7dbcbe-dcb6-11e2-8e2d-ae82c2f12506.png)