When using WP_Query's `"fields" => "ids"` (or `"fields" => "id=>parent"`), the returned values should be an array of integers, not array of integers represented by strings.

Adds unit tests. All other unit tests pass.

Props danielbachhuber.
Fixes #27252.



git-svn-id: https://develop.svn.wordpress.org/trunk@27686 602fd350-edb4-49c9-b593-d223f7449a82