Use correct ItemContainerGenerator for menus.

src/Avalonia.Controls/Menu.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Linq;
 using System.Reactive.Disposables;
+using Avalonia.Controls.Generators;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
@@ -131,6 +132,12 @@ namespace Avalonia.Controls
             _subscription.Dispose();
         }
 
+        /// <inheritdoc/>
+        protected override IItemContainerGenerator CreateItemContainerGenerator()
+        {
+            return new ItemContainerGenerator<MenuItem>(this, MenuItem.HeaderProperty, null);
+        }
+
         /// <summary>
         /// Called when a key is pressed within the menu.
         /// </summary>

src/Avalonia.Controls/MenuItem.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Linq;
 using System.Windows.Input;
+using Avalonia.Controls.Generators;
 using Avalonia.Controls.Mixins;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
@@ -204,6 +205,12 @@ namespace Avalonia.Controls
             IsSelected = true;
         }
 
+        /// <inheritdoc/>
+        protected override IItemContainerGenerator CreateItemContainerGenerator()
+        {
+            return new ItemContainerGenerator<MenuItem>(this, MenuItem.HeaderProperty, null);
+        }
+
         /// <summary>
         /// Called when a key is pressed in the <see cref="MenuItem"/>.
         /// </summary>