display featured image in RSS feed

function add_featured_image_url($output) {
    global $post;
    if ( has_post_thumbnail( $post->ID ) ){
        $output = wp_get_attachment_url( get_post_thumbnail_id( $post->ID ) ) . ' ' . $output;
    }
    return $output;
}

add_filter('the_excerpt_rss', 'add_featured_image_url');
add_filter('the_content_feed', 'add_featured_image_url');