Istio 1.0: HTTPRoute `appendHeaders` got wrong result if that header already exists in request headers.

echo-server.yaml

apiVersion: v1
kind: Namespace
metadata:
  name: istio-test
  labels:
    istio-injection: enabled
---
apiVersion: v1
kind: Service
metadata:
  name: echo-istio
  namespace: istio-test
spec:
  type: ClusterIP
  ports:
    - port: 8080
      targetPort: http
      protocol: TCP
      name: http
  selector:
    app: echo-istio
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: echo-istio
  namespace: istio-test
  labels:
    app: echo-istio
spec:
  selector:
    matchLabels:
      app: echo-istio
  replicas: 3
  template:
    metadata:
      labels:
        app: echo-istio
    spec:
      containers:
      - name: echo
        image: gcr.io/google-containers/echoserver:1.10
        ports:
        - containerPort: 8080
          name: http
---
apiVersion: networking.istio.io/v1alpha3
kind: Gateway
metadata:
  namespace: istio-test
  name: echo-gateway
spec:
  selector:
    istio: ingressgateway # use Istio default gateway implementation
  servers:
  - port:
      number: 80
      name: http
      protocol: HTTP
    hosts:
    - "*"
---
apiVersion: networking.istio.io/v1alpha3
kind: VirtualService
metadata:
  namespace: istio-test
  name: echo-virtual-service
spec:
  hosts:
  - "*"
  gateways:
  - echo-gateway
  http:
  - match:
    - uri:
        prefix: /echo-istio
    route:
    - destination:
        host: echo-istio
    appendHeaders:
      x-foo: bar