TUN-4947: Use http when talking to Unix sockets origins

Right now the proxying of cloudflared -> unix socket is a bit of
a no man's land, where we do not have the ability to specify the
actual protocol since the user just configures "unix:/path/"

In practice, we proxy using an HTTP client.
But it could be that the origin expects HTTP or HTTPS. However,
we have no way of knowing.

So how are we proxying to it? We are configuring the http.Request
in ways that depend on the transport and edge implementation, and
it so happens that for h2mux and http2 we are using a http.Request
whose Scheme is HTTP, whereas for quic we are generating a http.Request
whose scheme is HTTPS.

Since it does not make sense to have different behaviours depending
on the transport, we are making a (hopefully temporary) change so
that proxied requests to Unix sockets are systematically HTTP.

In practice we should do #502
to make this configurable.

Author: nmldiegues

ingress/origin_proxy.go

@@ -23,6 +23,7 @@ type StreamBasedOriginProxy interface {
 }
 
 func (o *unixSocketPath) RoundTrip(req *http.Request) (*http.Response, error) {
+	req.URL.Scheme = "http"
 	return o.transport.RoundTrip(req)
 }