TLS redirect_cleartext_from doesn't preserve path

Describe the bug url path is not preserved with redirect_cleartext_from set

To Reproduce

1. Follow TLS Termination documentation to create cert and store as kubernetes secret

2. Deploy ambassador with helm chart 2.2.1 with values:

service:
  annotations:
    getambassador.io/config: |
      ---
      apiVersion: ambassador/v1
      kind: Module
      name: tls
      config:
        server:
          enabled: True
          secret: ambassador-certs
          redirect_cleartext_from: 8080

1. Deploy httpbin service to test redirect

---
apiVersion: v1
kind: Service
metadata:
  name: httpbin
  annotations:
    getambassador.io/config: |
      ---
      apiVersion: ambassador/v1
      kind:  Mapping
      name:  httpbin_mapping
      prefix: /httpbin/
      service: httpbin.org:80
      host_rewrite: httpbin.org
spec:
  ports:
  - name: httpbin
    port: 80

1. curl the endpoint using http

curl -Li http://hostname/httpbin/

1. Result: path is not preserved on redirect

HTTP/1.1 301 Moved Permanently
location: https://hostname/
date: Wed, 24 Apr 2019 20:12:19 GMT
server: envoy
content-length: 0

HTTP/2 404 
date: Wed, 24 Apr 2019 20:12:19 GMT
server: envoy

Expected behavior Path should be preserved and redirect to https://hostname/httpbin/

Versions (please complete the following information):

• Ambassador: [0.60.0] (using Helm chart 2.2.1)
• Kubernetes environment: [AKS]
• Version [1.12.7]

Additional context

[gsagula]

The problem is that we shouldn’t be setting "path_redirect": "/" if the intention is to preserve the path in the request. There is a logic in Envoy for that:

if (!path_redirect_.empty()) {
    final_path = path_redirect_.c_str();
  } else {
    ASSERT(headers.Path());
    final_path = headers.Path()->value().getStringView();
    if (strip_query_) {
      size_t path_end = final_path.find("?");
      if (path_end != absl::string_view::npos) {
        final_path = final_path.substr(0, path_end);
      }
    }
  }

I will open a PR shortly with the fix. Thanks!

[gsagula]

@bpehling @christianhuening Thanks for the info. I will need to debug Envoy.