Allow to be run without middleware + improve request reading consistency (#217)

Prior to this change, the request URI was only ever read from the
X-Forwarded-Uri header which was only set when the container was
accessed via the forwardauth middleware. As such, it was necessary
to apply the treafik-forward-auth middleware to the treafik-forward-auth
container when running auth host mode.
This is a quirk, unnecessary complexity and is a frequent source of
configuration issues.

internal/server.go

@@ -58,7 +58,11 @@ func (s *Server) RootHandler(w http.ResponseWriter, r *http.Request) {
 	// Modify request
 	r.Method = r.Header.Get("X-Forwarded-Method")
 	r.Host = r.Header.Get("X-Forwarded-Host")
-	r.URL, _ = url.Parse(r.Header.Get("X-Forwarded-Uri"))
+
+	// Read URI from header if we're acting as forward auth middleware
+	if _, ok := r.Header["X-Forwarded-Uri"]; ok {
+		r.URL, _ = url.Parse(r.Header.Get("X-Forwarded-Uri"))
+	}
 
 	// Pass to mux
 	s.router.ServeHTTP(w, r)