Keep the reference icons when opening references

fixes https://github.com/Microsoft/visualfsharp/issues/559
closes https://github.com/Microsoft/visualfsharp/pull/623

vsintegration/src/vs/FsPkgs/FSharp.Project/Common.Source.CSharp/Project/ReferenceContainerNode.cs

@@ -130,7 +130,7 @@ public override string GetEditLabel()
 
         public override object GetIconHandle(bool open)
         {
-            return this.ProjectMgr.ImageHandler.GetIconHandle(open ? (int)ProjectNode.ImageName.OpenReferenceFolder : (int)ProjectNode.ImageName.ReferenceFolder);
+            return this.ProjectMgr.ImageHandler.GetIconHandle((int)ProjectNode.ImageName.ReferenceFolder);
         }