Fix serving files with unicode filename (e.g. "Pépé à la plage.jpg").

I am not sure I resolved this issue properly, this is why I put this in dev branch.

However my personal website is using it and it works.

Test stack:
Ubuntu 13.10 server 64 bits
Nginx 1.4.1-3ubuntu1.3
uWSGI 1.9.13-4build1
Python 3.3.2+
Django 1.8.dev20140418111815
My crazy code