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

however it would be good to create a test case in phpcr-api-tests for this too so i created a ticket for that.