throw a constraint violation when modifying a protected property

fix for https://github.com/doctrine/phpcr-odm/blob/master/tests/Doctrine/Tests/ODM/PHPCR/Functional/MixinTest.php#L77 with Jackalope Doctrine DBAL

with this fix we are down to only one failing test in PHPCR ODM. however it would be good to create a test case in phpcr-api-tests for this too.