CORS headers are not added on 4xx and 5xx responses

[gviot]

Is this a request for help? (If yes, you should use our troubleshooting guide and community support channels, see https://kubernetes.io/docs/tasks/debug-application-cluster/troubleshooting/.):
NO

What keywords did you search in NGINX Ingress controller issues before filing this one? (If you have found any duplicates, you should instead reply there.):
CORS

---

Is this a BUG REPORT or FEATURE REQUEST? (choose one):
BUG REPORT

NGINX Ingress controller version:
0.9-beta.16

Kubernetes version (use kubectl version):
Client Version: version.Info{Major:"1", Minor:"8", GitVersion:"v1.8.2", GitCommit:"bdaeafa71f6c7c04636251031f93464384d54963", GitTreeState:"clean", BuildDate:"2017-10-24T19:48:57Z", GoVersion:"go1.8.3", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"7+", GitVersion:"v1.7.8-gke.0", GitCommit:"a7061d4b09b53ab4099e3b5ca3e80fb172e1b018", GitTreeState:"clean", BuildDate:"2017-10-10T18:48:45Z", GoVersion:"go1.8.3", Compiler:"gc", Platform:"linux/amd64"}

Environment:

• Cloud provider or hardware configuration: GKE
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

What happened:
When enabling CORS headers, they are only added with status 200 responses, not 4xx or 5xx

What you expected to happen:
Headers should be added no matter what the status code is

How to reproduce it (as minimally and precisely as possible):

• create an nginx controller with version 0.9-beta.16
• create an ingress with the following annotations:
  kubernetes.io/ingress.class: "nginx"
  ingress.kubernetes.io/enable-cors: "true"
  ingress.kubernetes.io/cors-allow-headers: "DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Authorization"
• send a request to a backend that will answer with 500 status

Anything else we need to know:
SO issue with some info: https://stackoverflow.com/questions/20414669/nginx-add-headers-when-returning-400-codes
it seems that the nginx version used in the project supports adding "always" as a third parameter to add_header, this might fix the issue