Port all remaining self.protected_instance_variables to class methods
Merge pull request #12062 from strzalek/default-protected-vars

Port all remaining self.protected_instance_variables to class methods
pk should not be required for hm:t associations
make sure there are actually some categories when running the test
Fix broken link to Fixtures documentation on guides
and activerecord tests

[ci skip]
Remove not necessary AR changelog entry, fix markup [ci skip]
Stop messing up with instance variables, use protected as it was meant for
Merge pull request #12076 from dabit/master

Fix broken link to Fixtures documentation on guides and activerecord tests [ci skip]