Make GetObservable return current value.

Perspex.Base/PerspexObject.cs

@@ -295,7 +295,7 @@ namespace Perspex
                 {
                     this.PropertyChanged -= handler;
                 };
-            });
+            }).StartWith(this.GetValue(property));
         }
 
         /// <summary>

Perspex.Controls/Primitives/ScrollBar.cs

@@ -28,8 +28,6 @@ namespace Perspex.Controls.Primitives
 
         public ScrollBar()
         {
-            this.Classes.Add(":vertical");
-
             this.GetObservable(OrientationProperty).Subscribe(o =>
             {
                 if (o == Orientation.Horizontal)