🐛 BUG: Non-deterministic DownloadController unit test failure

[torss]

An apparent unit test bug manifested itself here in a Travis CI run:

1) DownloadFile
       POST /api/download/pdf/individual
         should pass:
     Error: Timeout of 30000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves. (/home/travis/build/geli-lms/geli/api/build/test/integration/download.js)

Re-running the tests (tested once locally and once in Travis CI) didn't reproduce this failure, so apparently this unit test bug is non-deterministic. Because of this it may be necessary to manually audit the unit test(s) and/or controller route code to find the problem.

Acceptance criteria:

• Find the cause of this issue.
  (Also check whether it might affect other DownloadController unit tests / routes.)
• Fix it.

Additional info:

This might be a duplicate of #913?