Customize: Ensure multiple CSS classes are passed to nav_menu_link_attributes as an array

When multiple CSS classes are added to a menu item, the nav_menu_link_attributes
filter should be called with $item->classes set to an array of CSS class names.

When previewing in the Customizer, however, a single string was being passed to
$item->classes because WP_Customize_Nav_Menu_Item_Setting::preview() bypasses
wp_update_nav_menu_item() and instead uses filter_wp_get_nav_menu_items().

The fix is to make filter_wp_get_nav_menu_items() match what
wp_update_nav_menu_item() does and split the string into an array.

Fixes #43113.
Props dlh.

Built from https://develop.svn.wordpress.org/trunk@49624


git-svn-id: http://core.svn.wordpress.org/trunk@49362 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/customize/class-wp-customize-nav-menu-item-setting.php

@@ -605,6 +605,11 @@ class WP_Customize_Nav_Menu_Item_Setting extends WP_Customize_Setting {
 			$item->post_title = $item->title;
 		}
 
+		// 'classes' should be an array, as in wp_setup_nav_menu_item().
+		if ( isset( $item->classes ) && is_scalar( $item->classes ) ) {
+			$item->classes = explode( ' ', $item->classes );
+		}
+
 		$item->ID    = $this->post_id;
 		$item->db_id = $this->post_id;
 		$post        = new WP_Post( (object) $item );

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-beta4-49623';
+$wp_version = '5.6-beta4-49624';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.