Fix date_done set when state changes back to pending

When Job.date_done has been set, for instance because the job has been
performed, if the job is set back to pending (e.g. a RetryableJobError
is raised), the date_done is kept.

Author: Guewen Baconnier

queue_job/job.py

@@ -594,6 +594,7 @@ def set_pending(self, result=None, reset_retry=True):
         self.state = PENDING
         self.date_enqueued = None
         self.date_started = None
+        self.date_done = None
         if reset_retry:
             self.retry = 0
         if result is not None: