non-latin support

[lopatoid]

I've had problems with Cyrillic filenames when creating FileDownloader object, and I was able to fix it just by changing line 701 in server.py to
self.send_header(k, urllib.parse.quote_plus(headers[k]))

[dddomodossola]

@lopatoid Thank you a lot for reporting. Can you please create a pull request to fix this?

[lopatoid]

My first solution was naive and just wrong. Anyway, I've found a detailed description of how to fix this problem on StackOverflow and made a pull request.