Themes: Add singular to the list of body classes when viewing a single post object.

Adds tests

Fixes #35164
Props danielpataki, johnbillion

Built from https://develop.svn.wordpress.org/trunk@36112


git-svn-id: http://core.svn.wordpress.org/trunk@36077 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/post-template.php

@@ -578,6 +578,9 @@ function get_body_class( $class = '' ) {
 		$classes[] = 'attachment';
 	if ( is_404() )
 		$classes[] = 'error404';
+	if ( is_singular() ) {
+		$classes[] = 'singular';
+	}
 
 	if ( is_single() ) {
 		$post_id = $wp_query->get_queried_object_id();