ModifyAttributes needs to pay attention to possible UserData changes

[sbaer]

ModifyAttributes is not recognizing changes to userdata on object attributes.

Rhino.Input.Custom.GetObject go = new Rhino.Input.Custom.GetObject();
go.Get();
Rhino.DocObjects.ObjectAttributes atts = go.Object(0).Object().Attributes;
atts.SetUserString("Jon", "Jon");
Guid id = go.Object(0).ObjectId;
Rhino.RhinoDoc.ActiveDoc.Objects.ModifyAttributes(id, atts, false);
string s = Rhino.RhinoDoc.ActiveDoc.Objects.Find(id).Attributes.GetUserString("Jon");
if(string.IsNullOrEmpty(s))
    Rhino.RhinoApp.Write("NoUserString" + "\n");
else
    Rhino.RhinoApp.Write(s + "\n");