Fix setting host header in action proxy in VirtualServer/VirtualServerRoute

[pleshakov]

Proposed changes

Previously, a configuration like below will make NGINX send two host headers to the backend - the default "$host" and the configured "bar.example.com":

apiVersion: k8s.nginx.org/v1
kind: VirtualServer
metadata:
  name: foo 
spec:
  host: foo.example.com
  upstreams:
  - name: foo
    port: 8080
    service: backend-svc
  routes:
  - path: "/"
    action:
      proxy:
        upstream: foo
        requestHeaders:
          set:
          - name: Host
            value: bar.example.com

Now NGINX will only send one host header: by default - "$host", or the one configured in requestHeaders

I also updated some of the tests to print diffs. Otherwise, it was hard to see the mismatch in results.