[Feature] cache headers for images

__Tasks:__

- [x] fix vhost headers for cached files
- [x] add Testcase
- [x] gather feedback for my changes
- [x] submit changes to the documentation

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | Feature #99
| BC Breaks     | none
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/500-assets/DET-509-Cache-Headers.md