Update wp_nav_menu to support relative URLs

[parisholley]

in wp-includes/nav-menu-template.php add:

363: $relative_url = untrailingslashit($_SERVER['REQUEST_URI']);

modify the in_array check to add that as an option

if ( in_array( $item_url, array( $current_url, $_indexless_current, $relative_url ) ) ) 

I encourage clients to use relative url's so they can test with navigation across environments regardless of domains.

No issues here.

duplicate #15954