When `meta_type` is passed with `orderby => meta_value`, `orderby` must also use `CAST()` to avoid scenarios like: `SELECT`ing by `UNSIGNED` and then ordering by `CHAR`. Adds unit test.

Fixes #21621.


git-svn-id: https://develop.svn.wordpress.org/trunk@25255 602fd350-edb4-49c9-b593-d223f7449a82