post_author not updated properly if assigning a new WP user

[dkoo]

If updating a post's author and assigning a WP user (not a CAP guest author), the post_author field isn't updated even though the author taxonomy term is updated. Because the term is updated properly, the post will render with the correct author byline, but the post will not appear in any WP queries targeting the post_author ID or name. Verified in v3.5.6.

To replicate:

1. Publish a post assigned to the site's default author.
2. Using the CAP sidebar UI, add another WP user (not guest author) and remove the original author. Update the post.
3. View the post and observe that the new WP user is shown as the byline.
4. However, using WP CLI, get the post ($ wp post get <ID>) and observe that the post_author field still shows the default author's ID, not the second WP user's.

[oscarssanchez]

I couldn't reproduce this locally. For me it is updating the post_author field with the correct ID as long as the author exists as a WP User.

[dkoo]

I'm still able to replicate locally and on other staging sites. On some sites I had to repeat step 2 with another WP user before the post_author field got "stuck", but it's easily reproducible.

[GaryJones]

I've been able to replicate this. So far, the fix in #988 seems to be working.