TestIssue and TestPullRequest both have field 'labels'

When working on SKARA-1587, I was trying to remove a label in a TestPullRequest but failed. After investigation, I found that TestPullRequest and TestIssue both have the field 'labels', so when I was trying to remove a label, it will just set TestIssue.labels to null instead of TestPullRequest.labels and this leads to the bug.