multi headers is be override in knative ingress

[wu0407]

knative version: v1.1.0 net-istio version: v1.1.0

1. create knative ingress:

   apiVersion: networking.internal.knative.dev/v1alpha1
   kind: Ingress
   metadata:
   annotations:
     networking.knative.dev/ingress.class: istio.ingress.networking.knative.dev
   name: url-1
   namespace: default
   spec:
   httpOption: Enabled
   rules:
   - hosts:
   - url.com
   http:
     paths:
     - headers:
         version:
           exact: canary
         abc:
           exact: abc
       path: /url-1
       splits:
       - appendHeaders:
           Knative-Serving-Namespace: default
           Knative-Serving-Revision: example-nginx-00006
         percent: 100
         serviceName: example-nginx-00006
         serviceNamespace: default
         servicePort: 80
   visibility: ExternalIP

2. net-istio will create virtualservice:

   apiVersion: networking.istio.io/v1beta1
   kind: VirtualService
   metadata:
   annotations:
   kubectl.kubernetes.io/last-applied-configuration: |
     {"apiVersion":"networking.internal.knative.dev/v1alpha1","kind":"Ingress","metadata":{"annotations":{"networking.knative.dev/ingress.class":"istio.ingress.networking.knative.dev"},"name":"url-1","namespace":"default"},"spec":{"httpOption":"Enabled","rules":[{"hosts":["url.com"],"http":{"paths":[{"headers":{"version":{"exact":"canary"},"abc":{"exact":"abc"}},"path":"/url-1","splits":[{"appendHeaders":{"Knative-Serving-Namespace":"default","Knative-Serving-Revision":"example-nginx-00006"},"percent":100,"serviceName":"example-nginx-00006","serviceNamespace":"default","servicePort":80}]}]},"visibility":"ExternalIP"}]}}
   networking.knative.dev/ingress.class: istio.ingress.networking.knative.dev
   creationTimestamp: "2022-01-12T09:15:03Z"
   generation: 29
   labels:
   networking.internal.knative.dev/ingress: url-1
   name: url-1-ingress
   namespace: default
   ownerReferences:
   - apiVersion: networking.internal.knative.dev/v1alpha1
   blockOwnerDeletion: true
   controller: true
   kind: Ingress
   name: url-1
   uid: b731f15b-6e9d-421c-86d0-078957f62dfe
   spec:
   gateways:
   - knative/knative-ingress-gateway
   hosts:
   - url.com
   http:
   - headers:
     request:
       set:
         K-Network-Hash: 4def9432b3a05cbe2f2d5a38cd88f1884bbe01509b41ee01272a685e05b38b00
   match:
   - authority:
       prefix: url.com
     gateways:
     - knative/knative-ingress-gateway
     headers:
       K-Network-Hash:
         exact: override
     uri:
       prefix: /url-1
   retries: {}
   route:
   - destination:
       host: example-nginx-00006.default.svc.cluster.local
       port:
         number: 80
     headers:
       request:
         set:
           Knative-Serving-Namespace: default
           Knative-Serving-Revision: example-nginx-00006
     weight: 100
   - match:
   - authority:
       prefix: url.com
     gateways:
     - knative/knative-ingress-gateway
     headers:
       abc:
         exact: abc
     uri:
       prefix: /url-1
   retries: {}
   route:
   - destination:
       host: example-nginx-00006.default.svc.cluster.local
       port:
         number: 80
     headers:
       request:
         set:
           Knative-Serving-Namespace: default
           Knative-Serving-Revision: example-nginx-00006
     weight: 100

3. The first header version: canary is to be override.

related code: https://github.com/knative-sandbox/net-istio/blob/ab9dfc86f07f3ce0da06f3cc42282fcf2e0ad82c/pkg/reconciler/ingress/resources/virtual_service.go#L264-L272

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[zhouguangming]

+1 @igsong