Fix regression in download redirect with unicode filenames.

Fixes an encoding problem when the filename is unicode:

```
Traceback (innermost last):

Module ZPublisher.Publish, line 60, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 46, in call_object
Module ftw.file.content.file, line 85, in download
Module ftw.file.browser.download, line 16, in __call__
Module urllib, line 1282, in quote
KeyError: u'\xfc'
```