Fixed scheduler related tests on Py3k.

This fixes the remaining scheduler related test failures on Python 3.

I also made a change to ensure that ``job.id`` always returns a ``str`` (it may previously return a byte string which causes tests to fail). I think this is the right approach and makes RQ more predictable to work with. But am not an expert in this domain so let me know if I'm mistaken.

The related changes are [here](https://github.com/selwin/rq/compare/scheduler-integration?expand=1#L0R196) and [here](https://github.com/selwin/rq/compare/scheduler-integration?expand=1#L0R226).