Checklist

• I have included the output of celery -A proj report in the issue.
  4.0.2, also master
• I have verified that the issue exists against the master branch of Celery.
  ^ based on code review, code hasn't changed

Steps to reproduce

create a project that has scheduled tasks, do --scheduler celery.beat.Scheduler -B instead of -B when starting a worker

Expected behavior

get running periodic tasks

Actual behavior

periodic tasks are not run at all.

The reason for this is relatively obvious: https://github.com/celery/celery/blob/master/celery/beat.py#L458

The beat_schedule is merged in only in PersistentScheduler and not the superclass.

in my own workaround 'FixedScheduler', I simply call self.merge_inplace(self.app.conf.beat_schedule) and then Scheduler's own setup_schedule.