fix undefined method `shift' error on migration

Fixes an error I was having during the 6.0 -> 7.1 migration on a long running GitLab instance:

```
undefined method `shift' for #<Issue::ActiveRecord_Relation:0x007f29837d1050>
```

I got the solution from https://gitlab.com/gitlab-org/gitlab-ce/issues/380.

I'm unsure how I can check to make sure this is completely correct and that the migration went as expected however I am reloading my 6.0 install to make sure the data between the 6.0 and 7.1 install appears correct.

Looks like this should be ported back to each of the following so that users don't have errors during the upgrade to 7.1:

v7.1.1 v7.1.0 v7.0.0 v6.9.2 v6.9.1 v6.9.0
@bbodenmiller 

> are you saying like `items = klass.of_projects(project_id).where(iid: iid).to_a` and drop the `items.is_a? Array` below?

yes

> I haven't figured out how to trigger this migration to test it

create 2 records with same iid bypassing rails validation. And yes - I need confirmation from you that it works before merge.

> Lastly how can I trigger Ruby to run this migration again after I edit it?

use `rake db:rollback` to revert migration and `rake db:migrate` ti run it again.