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
@randx are you saying like `items = klass.of_projects(project_id).where(iid: iid).to_a` and drop the `items.is_a? Array` below? Works for me but I can't say I've wrote a bunch of Ruby so unsure if that is proper. Additionally I haven't figured out how to trigger this migration to test it, any steps to produce a scenario where this migration needs to run? Lastly how can I trigger Ruby to run this migration again after I edit it?