Ensure that get_archive_template() only loads a post type archive...

Ensure that get_archive_template() only loads a post type archive (archive-$post_type.php file) if there is exactly one post type in the query.

props scribu.
see #22956.
for trunk.



git-svn-id: https://core.svn.wordpress.org/trunk@23249 1a063a9b-81f0-0310-95a4-ce76da25c4cd