Retain the publishing date when updating posts.

[peterwilsoncc]

Description of the Change

When updating a published post, the post_date and post_date_gmt fields should remain unchanged.

Closes #700

How to test the Change

1. On a multisite install, create two sites
2. From the primary site publish a post
3. Push distribute the post to a secondary site (published, not as draft).
4. Visit the post on the front end of second site and observe the published time.
5. Wait at least 60 seconds
6. Edit the distributed post in the second site's admin.
7. Unlink the post.
8. Relink the post.
9. Visit the post on the front end of the second site and observe the publish time.

On develop the publish time will have changed after relinking the post.
On this branch the publish time will not change.

Changelog Entry

Fixed - Retain published time when editing distributed posts.

Credits

Props @madriverweb, @codiller, @peterwilsoncc

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[Sidsector9]

Thanks for working on this, tests well for me 👍