Add a "OnUnpublish" event to content handlers

Work Item: 16856

--HG--
branch : dev

src/Orchard.Web/Modules/Orchard.Indexing/Handlers/CreateIndexingTaskHandler.cs

@@ -23,6 +23,7 @@ namespace Orchard.Indexing.Handlers {
             _indexNotifierHandlers = indexNotifierHandlers;
 
             OnPublished<ContentPart>(CreateIndexingTask);
+            OnUnpublished<ContentPart>(CreateIndexingTask);
             OnRemoved<ContentPart>(RemoveIndexingTask);
         }