Add search_columns arg to WP_User_Query to allow for explicit column choices....

Add search_columns arg to WP_User_Query to allow for explicit column choices. Without it, the columns will be detected based on the search term. see #19810.

git-svn-id: https://core.svn.wordpress.org/trunk@19882 1a063a9b-81f0-0310-95a4-ce76da25c4cd