[BUGFIX] fixed preview bug with websockets

Makes the live content preview workable in combination with the websocket

__Tasks:__

- [x] test coverage
- [x] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | Preview in ajax mode can't return other formats than html anymore
| Doc           | none