Fix JIRA mention spec

Merged Douwe Maan requested to merge fix-jira-mention-spec into master

Fixes spec that was skipped in 10de4261.

The expected behavior with commits mentioning issues, is that when the commit closes the issue, the issue only gets the "closed by commit X" comment, and not also the "mentioned in commit X" comment.

With JIRA, it used to get both, and now does no longer.

The spec was updated to test the new ("correct") behavior.