Resolve MTurkUnit expiration bugs

[JackUrb]

Overview

A combination of two bugs was leading to improper outcomes when expiring a task with Ctrl-C. First, MTurkUnit wasn't properly updating from the CREATED status to the ASSIGNED status unless a db status load occurred elsewhere. The second issue was that moving from ASSIGNED to EXPIRED didn't trigger the same return event that it did when moving from ASSIGNED to LAUNCHED, preventing running assignments from marking and exiting properly. The combination of these two bugs led to some completed tasks being locally marked as expired, and preventing some workers from being able to submit complete work after a Ctrl-C was triggered.

Resolution

To resolve the lack of CREATED state change, MTurkUnit's get_status method now calls the super method in when currently in CREATED, forcing that refresh using the base logic.
For triggering a RETURNED event, the logic for this is extended to include the transition from ASSIGNED to EXPIRED, however the new transition should still set the unit's db status (rather than deferring to the blueprint to decide whether to expire or make available again) as we know the outcome is EXPIRED.

Testing

Launched tasks in MTurk sandbox, noted I could now get a proper return event after expiring hits, and could properly exit on a double-return event. Unit tests still passing

[jxmsML]

👍 🚀 thanks for fixing this so fast!!

[jxmsML]

is this one good to merge?