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

This reverts commit 6e3ab3e15faf782f6a937ccf5574a4fb63e3e353, reversing
changes made to 39e07b64ce3f4bb55e60ba0266e677f8e4f4893a.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/cases/autosave_association_test.rb