add Test for several inehritance level
update Inheritance2Test
Update Inheritance2Test.php
Update Inheritance2Test.php
Update Inheritance2Test.php
Update Inheritance2Test.php
[tests] update sluggable inheritance test
[tests] sluggable inheritance issue
[mapping] provide a way to get root class name from adapter
[sluggable] fix inheritance bug, when persisting multiple slugs, closes #667