Bugfix: correct referrer for report-a-problem

For problem reports with an unknown referrer submitted via AJAX,
[the 'referrer' param is set to 'unknown'][1]. This case was broken
by the changes introduced by https://github.com/alphagov/feedback/pull/109.

[1]: https://github.com/alphagov/static/blob/master/app/assets/javascripts/report-a-problem.js#L64