WP_Query: If the post_type QV is an array with only one element, don't treat it as a multiple post type query.

If there is only one post type used by all taxonomies for a tax query, set the post type to the string.

fixes #24204.



git-svn-id: https://core.svn.wordpress.org/trunk@24115 1a063a9b-81f0-0310-95a4-ce76da25c4cd