host_redirect cleans up the request port when port_redirect is not specified

Description: When host_redirect is specified but port_redirect is not, port is not preserved in the response. Envoy cleans up the port in the response(https://github.com/envoyproxy/envoy/blob/main/source/common/router/config_impl.cc#L791). Envoy should preserve the port in the request if port_redirect is not specified or it should be clarified in the doc(https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/route/v3/route_components.proto#envoy-v3-api-msg-config-route-v3-redirectaction) that the port will be cleaned up if port_redirect is not specified.

Repro steps:

• Configure a route to do host_redirect in a listener that listens to a port other than 80 or 443
• Send a request to the route

curl -v --resolve redirect-test:10080:127.0.0.1 http://redirect-test:10080
* Added redirect-test:10080:127.0.0.1 to DNS cache
* Rebuilt URL to: http://redirect-test:10080/
* Hostname redirect-test was found in DNS cache
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to redirect-test (127.0.0.1) port 10080 (#0)
> GET / HTTP/1.1
> Host: redirect-test:10080
> User-Agent: curl/7.52.1
> Accept: */*
> 
< HTTP/1.1 301 Moved Permanently
< location: http://redirect-test.mydomain.com/
< date: Tue, 13 Jul 2021 17:33:32 GMT
< server: envoy
< content-length: 0
< 
* Curl_http_done: called premature == 0
* Connection #0 to host redirect-test left intact

Config:

static_resources:
  listeners:
  - address:
      socket_address:
        address: 0.0.0.0
        port_value: 10080
    filter_chains:
    - filters:
      - name: envoy.http_connection_manager
        config:
          codec_type: auto
          stat_prefix: ingress_http
          route_config:
            name: local_route
            virtual_hosts:
            - name: backend
              domains:
              - "*"
              routes:
              - match:
                  prefix: "/"
                redirect:
                  host_redirect: "redirect-test.mydomain.com"
          http_filters:
          - name: envoy.router
            config: {}