Fix menu separators.

samples/ControlCatalog/Pages/MenuPage.xaml

@@ -10,6 +10,7 @@
       <Menu>
         <MenuItem Header="_First">
           <MenuItem Header="Standard _Menu Item"/>
+          <Separator/>
           <MenuItem Header="Menu with _Submenu">
             <MenuItem Header="Submenu _1"/>
             <MenuItem Header="Submenu _2"/>

src/Avalonia.Controls/Avalonia.Controls.csproj

@@ -51,6 +51,7 @@
     <Compile Include="DockPanel.cs" />
     <Compile Include="Expander.cs" />
     <Compile Include="Generators\ItemContainerInfo.cs" />
+    <Compile Include="Generators\MenuItemContainerGenerator.cs" />
     <Compile Include="Generators\TreeContainerIndex.cs" />
     <Compile Include="HotkeyManager.cs" />
     <Compile Include="IApplicationLifecycle.cs" />

src/Avalonia.Controls/Generators/MenuItemContainerGenerator.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Avalonia.Controls.Generators
+{
+    public class MenuItemContainerGenerator : ItemContainerGenerator<MenuItem>
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ItemContainerGenerator{T}"/> class.
+        /// </summary>
+        /// <param name="owner">The owner control.</param>
+        public MenuItemContainerGenerator(IControl owner)
+            : base(owner, MenuItem.HeaderProperty, null)
+        {
+        }
+
+        /// <inheritdoc/>
+        protected override IControl CreateContainer(object item)
+        {
+            var separator = item as Separator;
+            return separator != null ? separator : base.CreateContainer(item);
+        }
+    }
+}

src/Avalonia.Controls/MenuItem.cs

@@ -208,7 +208,7 @@ namespace Avalonia.Controls
         /// <inheritdoc/>
         protected override IItemContainerGenerator CreateItemContainerGenerator()
         {
-            return new ItemContainerGenerator<MenuItem>(this, MenuItem.HeaderProperty, null);
+            return new MenuItemContainerGenerator(this);
         }
 
         /// <summary>