Description:
------------
According to RFC 7231, the Date header is mandatory for HTTP responses. It should contain "the best available approximation of the date and time of message generation".

https://tools.ietf.org/html/rfc7231#section-7.1.1.2

Usually, it's the webserver's task to add this header. For instance, the header is magically present in responses coming from an Apache webserver with mod_php.

PHP's build-in webserver on the other hand does not add that header.

Strangely enough, it sends a Host header with the response (copied from the request), although that header is afaik a request-only header.

Test script:
---------------
php -S localhost:8080 any-script.php

Expected result:
----------------
GET / HTTP/1.1
Host: localhost

HTTP/1.1 200 OK
Date: Mon, 04 Jan 2016 16:36:54 GMT
Content-type: text/html; charset=UTF-8

<!-- The actual content -->

Actual result:
--------------
GET / HTTP/1.1
Host: localhost

HTTP/1.1 200 OK
Host: localhost
Connection: close
X-Powered-By: PHP/7.0.1
Content-type: text/html; charset=UTF-8

<!-- The actual content -->