Does not work with custom WP_CONTENT_DIR

I had a problem on a site, where modification_time() would just return false.

Turns out that this line:
$file = realpath( ABSPATH . str_replace( get_site_url(), '', $src ) );
Does not play well will installations with custom WP_CONTENT_DIR set (and WordPress installation in it’s own dir, like /wp/).

I changed it to
$file = realpath( $_SERVER["DOCUMENT_ROOT"] . wp_make_link_relative( $src ) );
and then it worked nicely on my sites.

If you could add support for sites with custom WP_CONTENT_DIR that would be awesome!

https://wordpress.org/plugins/busted/