Fix DataContext property for items controls.

Perspex.Controls/Generators/ItemContainerGenerator.cs

@@ -84,6 +84,7 @@ namespace Perspex.Controls.Generators
                 foreach (object item in items)
                 {
                     Control container = this.CreateContainerOverride(item);
+                    container.DataContext = item;
                     container.TemplatedParent = null;
                     this.AddInternal(item, container);
                     result.Add(container);

Perspex.Controls/Templates/DataTemplateExtensions.cs

@@ -30,8 +30,6 @@ namespace Perspex.Controls.Templates
                 result = DataTemplate.Default.Build(data);
             }
 
-            result.DataContext = data;
-
             return result;
         }