HTTPRoute filters need to apply the user provided value for the existing headers

[salonichf5]

Describe the bug
HTTPRoute filters does not honor the user provided value for already existing headers.

To Reproduce
Steps to reproduce the behavior:

Apply the gateway.yaml and headers application

use the following HTTPRoute

apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: headers
spec:
  parentRefs:
  - name: gateway
    sectionName: http
  hostnames:
  - "echo.example.com"
  rules:
  - matches:
    - path:
        type: PathPrefix
        value: /headers
    filters:
    - type: RequestHeaderModifier
      requestHeaderModifier:
        set:
        - name: X-Forwarded-Proto
          value: https
    backendRefs:
    - name: headers
      port: 80

The header X-Forwarded-Proto

Nginx Config

        proxy_http_version 1.1;
        proxy_set_header X-Forwarded-Proto "https";
        proxy_set_header Host "$gw_api_compliant_host";
        proxy_set_header X-Forwarded-For "$proxy_add_x_forwarded_for";
        proxy_set_header X-Real-IP "$remote_addr";
        proxy_set_header X-Forwarded-Proto "$scheme";
        proxy_set_header X-Forwarded-Host "$host";
        proxy_set_header X-Forwarded-Port "$server_port";
        proxy_set_header Upgrade "$http_upgrade";
        proxy_set_header Connection "$connection_upgrade";
        proxy_pass http://default_headers_80$request_uri;

The header is X-Forwarded-Proto set twice.

Expected behavior
We need to set the header X-Forwarded-Proto once to https (user provided value).

Your environment

• Version of the NGINX Gateway Fabric - release version or a specific commit. The first line of the nginx-gateway container logs includes the commit info. -- edge
• Version of Kubernetes
• Kubernetes platform (e.g. Mini-kube or GCP)
• Details on how you expose the NGINX Gateway Fabric Pod (e.g. Service of type LoadBalancer or port-forward)
• Logs of NGINX container: kubectl -n nginx-gateway logs -l app=nginx-gateway -c nginx
• NGINX Configuration: kubectl -n nginx-gateway exec <gateway-pod> -c nginx -- nginx -T

Additional context
Add any other context about the problem here. Any log files you want to share.