Revert "Revert "Merge pull request #8313 from alan/only_save_changed_has_one_objects""

This reverts commit e94e6c27af495a2460c811bb506459f1428dec6b.

Conflicts:
	activerecord/CHANGELOG.md

The original commit was reverted only to be safe since #14407 were reported.
We don't have any proof we added a regression with the original commit
so reverting it now will give us more problem.

Closes #14407