Support ETag and Last-Modified in Response

Docker Image used: 6c07143937e57095d8478efc8ab7eab52b44e67c7673285f8c0a2bf4a7b137ad

```
openapi: 3.0.3
info:
title: test
version: '1.0'
servers:
- url: 'http://localhost:8000'
paths:
/test:
get:
responses:
'200':
description: OK
content:
application/json:
schema:
type: object
properties:
ok:
type: boolean
default: 'true'
examples:
a:
value:
ok: true
headers:
ETag:
schema:
type: string
example: '"c561c68d0ba92bbeb8b0f612a9199f722e3a621a"'
Last-Modified:
schema:
type: string
example: Mon, 18 Jul 2016 02:36:04 GMT
```

Will return:

```
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json
Etag: string
Last-Modified: string
Date: Thu, 23 Apr 2020 22:38:43 GMT
Content-Length: 16
```

Either the example values should be provided or (better) real values should be generated.