[BUGFIX] phpcr value format change exception try to delete property and rewrite it

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | none
| Fixed tickets | #544
| BC Breaks     | no
| Doc           | none
A testcase for this was added in ContentMapperTest.php
internal-link(string) change to snippet (array) change back to internal-link(string)

When it was switched back to internal-link(value is array because before it was a snippet) we need to check if the given data is a string, before json_decode in read function of Internal Link is called.