Fix default button press.

It was being invoked when enter pressed on menu item.

Perspex.Controls/Button.cs

@@ -203,7 +203,7 @@ namespace Perspex.Controls
 
         private void ListenForDefault(IInputElement root)
         {
-            root.AddHandler(InputElement.KeyDownEvent, this.RootKeyDown, RoutingStrategies.Tunnel);
+            root.AddHandler(InputElement.KeyDownEvent, this.RootKeyDown);
         }
 
         private void StopListeningForDefault(IInputElement root)

Perspex.Controls/MenuItem.cs

@@ -263,6 +263,7 @@ namespace Perspex.Controls
                     else
                     {
                         this.RaiseEvent(new RoutedEventArgs(ClickEvent));
+                        e.Handled = true;
                     }
 
                     break;