Allow custom status codes to be passed through from upstream

[kdorosh]

Workflow that should be possible

1. Install Gloo
2. Deploy upstream

apiVersion: gloo.solo.io/v1
kind: Upstream
metadata:
  name: postman-echo
  namespace: gloo-system
spec:
  static:
    hosts:
    - addr: postman-echo.com
      port: 80

3. Deploy VirtualService

apiVersion: gateway.solo.io/v1
kind: VirtualService
metadata:
  name: update-response-code
  namespace: gloo-system
spec:
  virtualHost:
    domains:
    - '*'
    routes:
    - matchers:
       - prefix: /
      routeAction:
        single:
          upstream:
            name: postman-echo
            namespace: gloo-system
      options:
        autoHostRewrite: true

4. Run curl -v -H "Content-Type: application/json" "$(glooctl proxy url)/get?foo1=bar1&foo2=bar2= ="

Note the status code:

< HTTP/1.1 400 Bad Request

This modified from the upstream, which for curl -v --location --request GET 'https://postman-echo.com/get?foo1=bar1&foo2=bar2= =' returns:

< HTTP/1.1 400 BAD_REQUEST

We should allow the original status codes to be passed through.

[GrahamGoudeau]

Note that this isn't piping through only the status code. The customer's use case is:

for example, instead of:
400 Bad Request
Our app actually returns:
400 Vehicle not Found

[kdorosh]

Correct, the http1 reason phrase needs to be preserved. This requires a change to upstream envoy, as the reason phrase is dropped by envoy for http2 compliance.

[kdorosh]

Related upstream envoy issue, similar to what we'd need to do: envoyproxy/envoy#8463

http1 codec needs to be updated: https://github.com/envoyproxy/envoy/blob/master/source/common/http/http1/codec_impl.cc

[kdorosh]

opted not to do this because the reason phrase should not be read by programs to make routing decisions