fix Location header key to Unicode

Location header key is unicode in case of relative redirects, breaks uwsgi.
See https://github.com/mitsuhiko/flask/issues/758

@mitsuhiko fixed the bug however reopened by @puzzlet's patch(https://github.com/mitsuhiko/werkzeug/commit/6049a4ff3c5224bedd24cdb29a23a5f1a9c2bf43)