Handle non-ascii headers in multipart/form-data

Second stab at shazow/urllib3#119, after I left #121 lying around for far too long. Now this is integrated with the RequestField infrastructure. If you want me to, I can add another test for the overall structure of a non-ASCII form submission later on. Basically copy gagern/urllib3@71f0ed22cda47dc80495ca10479564d07de5c933 here.