Perform partial update only when changing shadow status

https://github.com/sulu-cmf/sulu-standard/issues/253

__Tasks:__

- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | ?
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/253
| BC Breaks     | 
| Doc           |