Add forgotten doc
On the non required onetoone relations, we only validate the required children if at least one value is posted.
* Delete the old value from the DB for the onetoone field
* Improve the tests
If no value posted for onetoone field, we return None to delete the value from the DB
Test the non required onetoone field
Merge pull request #12 from LeResKP/feature/onetoone_relation

Improve the onetoone relation