Push nil scope when switching windows and frames to fix issue 1103

This pushes a nil scope onto the scopes stack when switching windows and frames, and then interprets that nil as document while current_scope is called.  This has the effect of changing the scope to be the document in the switched to frame or window.  Hopefully this works across multiple drivers, the other option would be to make each drivers within_frame and within_window responsible for updating the scope.