wrong stackless.current in schedule callback #42

Originally reported by: Anselm Kruis (Bitbucket: akruis, GitHub: akruis)

stackless.run() switches from the main tasklet to another tasklet T. If a schedule callback is invoked for this switch, the stackless.current is T. This is incorrect. Inspection of the code in slp_schedule_task_prepared() reveals, that Stackless calls the schedule callback before the switch.

I added a test case and I also have a preliminary fix.

• Bitbucket: https://bitbucket.org/stackless-dev/stackless/issue/42