Merge branch feature/product-rest-services into develop

* Fixed wrong handling of `state`, `type` & `template` in `ProductController`
* Changed testPost() (`state`, `type` & `template` handling)
* Added testPostWithParent()

- [x] test coverage
- [x] finish the code
- [x] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [yes]
| Tests pass?   | [yes]
| Fixed tickets | [none]
| Doc           | [none]