Fix incorrect global post object when field is fields => 'id=>parent for WP_Query.

[Juzar10]

The next_post function assumes a WP_Post object, which isn't always the case. When a standard object with ID and parent is provided, setup_postdata creates an empty WP_Post object with just the ID.

This PR resolves the issue by passing only the post ID from next_post when the object is not a WP_Post.

Trac ticket: https://core.trac.wordpress.org/ticket/56992

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Props juzar, peterwilsoncc, joemcgill.

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[joemcgill]

Rather than returning a possible int value when the current post isn't a WP_Post object, would it be better to hydrate a WP_Post object from the ID and return that instead?

[Juzar10]

@joemcgill, I agree with you, that would be more consistent. I have updated the logic to return WP_Post object.

[peterwilsoncc]

I've added a suggestion inline to expand the tests to include calls to ::next_post and to make sure requesting only the IDs returns the expected data.

Let me know what you think.

[peterwilsoncc]

Thanks @Juzar10, this change looks good to me.

I took the liberty of merging trunk in to your branch as some significant changes were committed to WP_Query in the last 24 hours so I wanted to make sure there weren't any conflicts.

[Juzar10]

thanks, @peterwilsoncc.