The ProxyHandler apparently assumes that you're running a gorilla.Mux
for request routing, as it sets the URL.Scheme field of the incoming
request - which is normally unset. This has the consequence of messing
up all sorts of things, including redirects, because the other
fields (like Host) of the request URL are still going to be unset.

Drop github.com/tstranex/u2f as the implementation, in favor of
github.com/duo-labs/webauthn.

The clients already support it (via the default net/http.Transport),
and it can be enabled with a configuration parameter. The default is
still currently disabled, though there is plan to flip it later to
enabled-by-default if it proves to be working as expected in
production.

We also introduce, via a slight refactoring of the Serve() code, a
test of the top-level http.Handler (with the full middleware stack).

Use mime.ParseMediaType to obtain the main content type.