KNL-1314 Fix Chrome's caching of PDFs.

When Chrome 39 loads a PDF if gets back the data but then if you attempt to reload the PDF it can't display it. This seems to be because we don't have Cache-control headers on our response served up from /access. Adding Cache-control headers fixes the problem.

git-svn-id: https://source.sakaiproject.org/svn/kernel/trunk@316006 66ffb92e-73f9-0310-93c1-f5514f145a0a