Update a HIT with a status of Reviewable to have a status of Reviewing, 
        or reverts a Reviewing HIT back to the Reviewable status.

        Only HITs with a status of Reviewable can be updated with a status of
        Reviewing.  Similarly, only Reviewing HITs can be reverted back to a
        status of Reviewable.

def set_reviewing(self, hit_id, revert=None):
        """
        Update a HIT with a status of Reviewable to have a status of Reviewing, 
        or reverts a Reviewing HIT back to the Reviewable status.

        Only HITs with a status of Reviewable can be updated with a status of
        Reviewing.  Similarly, only Reviewing HITs can be reverted back to a
        status of Reviewable.
        """
        params = {'HITId' : hit_id,}
        if revert:
            params['Revert'] = revert
        return self._process_request('SetHITAsReviewing', params)