Ensure scenarios invoked from scripts use the ScriptTable class of the calling script

When a custom subclass of ScriptTable is used and a scenario is invoked from the script. Currently the ScenarioTable class will always convert its contents into assertions using the standard ScriptTable class.

To me it seems more natural/expected that the ScenarioTable would use the table-class of the calling script. This view seems to be supported by the documentation (FitNesse.UserGuide.WritingAcceptanceTests.SliM.ScenarioTable) which states: "They are macros, not programs. They are constructed via text substitution." which to me indicates the type of script table used should not change when replacing lines of script with a scenario call.