Changing draft visibility doesn't dirty the draft

[nikclayton]

Describe the bug

Changing a draft's visibility and going back doesn't prompt to save.

To Reproduce
Steps to reproduce the behavior:

1. Create a post, back out to save as draft with public visibility
2. Open the newly created draft
3. Change visibility to private
4. Go back. There's no prompt to save. If the draft is re-opened it has the original ("public") visibility.

Expected behavior

Draft status is dirty, and should be prompted to save.

Versions
2.8.3