Fix authentication in Safari.

We now offer 'qop="auth"', because without it, authentication in
Safari fails immediately. With the field set to auth, authentication
seems to work in the latest Firefox, Safari, Chrome and Internet
Explorer. However, the HTTP package as used in cabal-install has a bug
where it send 'qop="auth"' without an 'nc' or 'cnonce' field. So we
are lenient: when these fields are not present, we fall back to no
qop.

Fixes #132.

[1] https://github.com/haskell/HTTP/issues/54