No way to transition post status to draft after scheduling without publishing first

[earnjam]

Describe the bug

If you schedule a post for the future, there is no way to unschedule or change it to a draft without publishing it first.

This is similar to #3772 but that appears to be resolved with relation to auto-saves. This occurs when you attempt to manually save/switch to a draft.

To Reproduce

1. Create a post
2. Schedule post for future date
3. Adjust the date to the past
4. Click "Save Draft"
5. It publishes the post and button text is changed to "Convert to Draft", signifying that it is published.

Expected behavior

Clicking "Save Draft" should transition the post status back to draft without publishing. Otherwise there is no point to having both a "Save Draft" button and a "Publish" button visible.

Screenshots

Classic Editor

Somewhat confusing because the button still says "Publish" if you switch the status back to draft, but it at least gives you the option of making it a draft.

Gutenberg

Additional context

• Gutenberg v3.4
• WP trunk