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.

examples/traefik-v2/kubernetes/advanced-separate-pod/traefik-forward-auth/ingress.yaml

@@ -16,7 +16,5 @@ spec:
     services:
     - name: traefik-forward-auth
       port: 4181
-    middlewares:
-      - name: traefik-forward-auth
   tls:
     certresolver: default