Fix creating issues by email with Service Desk on

What does this MR do?

The SD handler was too 'greedy' before, and the specs I added to check this case were wrong 😞

Are there points in the code the reviewer needs to double check?

Don't think so.

Why was this MR needed?

We broke emailing an issue to a project on GitLab.com 😱

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/31434.