Always set headers with add-headers option

[zerda]

What this PR does / why we need it:
With nginx-custom-headers configmap and add-headers option.

apiVersion: v1
data:
  X-Request-Id: $req_id
kind: ConfigMap
metadata:
  name: nginx-custom-headers
  namespace: default

apiVersion: v1
data:
  add-headers: default/nginx-custom-headers
kind: ConfigMap
metadata:
  name: nginx-configuration
  namespace: default

It should set X-Request-Id header to response all the time, but failed at following scenarios because of limits with add_header directive.

From the documentation on add_header

1. when response code is 40x or 50x.
   By default, add_header adds the specified field to a response header provided only when the response code equals 200, 201 (1.3.10), 204, 206, 301, 302, 303, 304, 307 (1.1.16, 1.0.13), or 308 (1.13.0).

2. when add_header used in location block
   These directives are inherited from the previous level if and only if there are no add_header directives defined on the current level.
   If an ingress hasnginx.ingress.kubernetes.io/auth-url annotation, it will not set X-Request-Id to the response, because the add_header Set-Cookie $auth_cookie; already exists in location.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged):
fixes #4359

Special notes for your reviewer:
None

[k8s-ci-robot]

Hi @zerda. Thanks for your PR.

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[aledbf]

/ok-to-test

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[aledbf]

/retest

[aledbf]

/lgtm

[aledbf]

@zerda thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, zerda

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