In `WP_Date_Query::get_sql_for_subquery()`, don't parse duplicate parameters - only parse one of `w` and `week` or `month` and `monthnum`.

Adds unit tests.

Props oso96_2000, ChriCo.
Fixes #25835.



git-svn-id: https://develop.svn.wordpress.org/trunk@28252 602fd350-edb4-49c9-b593-d223f7449a82