Only allow deletion of related models in nested mutations

[lucaslenz]

Resolves #2455

• [x] Added or updated tests
• [ ] Documented user facing changes
• [ ] Updated CHANGELOG.md

Changes

Add failing test cases for a bug concerning the delete field in nested mutations. Documentation says: delete the related model and the association to it (link). These failing test cases show that actually any object can be deleted no matter of the relationship status to the root object. This should not be possible!

Breaking changes

I am not aware of any breaking changes.

[spawnia]

Please resolve conflicts.

[spawnia]

Again, your branch is out of date. You need to add this repository as a remote to your project and merge/rebase against the latest master.

[spawnia]

The merge was totally messed up, I just fixed it.