add server header in middleware

[teeann]

Fix for #7

Steps to reproduce

1. sudo clone --target http://example.com
2. sudo snare --port 8080 --page-dir example.com
3. curl --head http://localhost:8080/notfound. Response:

HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 1237
Date: Mon, 01 Apr 2019 16:49:00 GMT
Server: Python/3.6 aiohttp/3.4.4

Server header above is not changed to default nginx

[rnehra01]

The response code is 200 while it should be 404. Can you raise an issue for that and look into that before merging this?

[teeann]

Sure, I agree with you @rnehra01