Extending an overdue reservation doesn't unset the overdue flag [v5.5]

[orenyk]

See #1449 for more details; steps to replicate:

1. Have an overdue reservation (due date prior to today, overdue attribute set to true)
2. Edit said reservation, change due date to be after today
3. Save reservation

What we want: overdue = false
What we get: overdue = true

I'll put together a quick patch for release-v5.5 and then a proper patch with tests for master (probably not today).

[orenyk]

Had to tweak a bunch of specs; now that we have a callback forcing the overdue attribute to reflect reality for checked out reservations we have to use update_columns to bypass said callback in tests (see here). Just thought it would be interesting to note.

[orenyk]

Ok, wrote some specs for this, issuing PR for master.