For a while, prior to 987536930, we did not null assigned_type on
checkin. This migration manually nulls all assigned_type fields if
assigned_to is unset. Add a test to AssetTest for this as well...kind
of. We need to extract an Asset::checkin() method for 4.1 that mirrors
Asset::checkOut() to really test this.
This also fixes a separate (but related) issue. The Asset importer did
not set assigned_type when importing and creating users. In this
instance, we assume that if assigned_to is set and assigned_type is not,
then the item was checked out to a user and update the DB accordingly.
Also add a check in ImporterTest for this issue.
a5764351f7