Patch to enable developers to use different orderings per field specifie...

I want a query ordered by menu order and date with both columns sorted in DESC order.

My code is:

    $query = new WP_Query(array(
	    'orderby'           => 'menu_order date',
	    'order'             => 'DESC',
    ));

But WP_Query creates this query (which MySQL sorts menu_order ASC because no column ordering was specified):

    ORDER BY wp_posts.menu_order,wp_posts.post_date DESC

This PR will create this query:  

    ORDER BY wp_posts.menu_order DESC,wp_posts.post_date DESC

Or if `'order' => 'DESC ASC'` it will create this query:

    ORDER BY wp_posts.menu_order DESC,wp_posts.post_date ASC