Fix label serialisation in issue and note hooks
What does this MR do?
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29384 was not my greatest review. We were not calling
hook_attrs on the labels correctly. Specs were
passing because the issues under test did not have any labels! I also fixed the issues hook while I was there as it had a similar issue.
Does this MR meet the acceptance criteria?
Performance and testing
- Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.