Wrap the filenames with quotes for the Content-Disposition header

So that filenames with commas don't cause "Duplicate headers received from server" error in Chromium-based browsers.

See also:
* https://groups.google.com/forum/#!topic/web2py/paweofnBz-g
* http://stackoverflow.com/questions/10120197/nginx-rails-send-file-duplicate-headers-received-from-server
* https://github.com/prior/prawnto/pull/16
* https://github.com/prior/prawnto/commit/fddf4fa257c62ca87cbcde0cdc449f9d1c084c6e