Customize: Ensure a newly-added custom link nav menu item has the appropriate CSS class names.

For nav menu items of the `custom` type, their `object` should also be set to `custom`, so that the `menu-item-custom` class name will be added.

Props vishalkakadiya.
Fixes #37575.


git-svn-id: https://develop.svn.wordpress.org/trunk@38301 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/customize-nav-menus.js

@@ -446,7 +446,7 @@
 				'url': itemUrl.val(),
 				'type': 'custom',
 				'type_label': api.Menus.data.l10n.custom_label,
-				'object': ''
+				'object': 'custom'
 			};
 
 			this.currentMenuControl.addItemToMenu( menuItem );