php_cli_server: ensure a single Date header is present

[coppolafab]

Currently the PHP Development Server appends a Date header in the response, despite already set from user code.

Steps to reproduce

index.php:
<?php header('Date: Mon, 25 Mar 1985 00:20:00 GMT');

terminal 1:
$ php -S localhost:8080 index.php

terminal 2:

$ curl -i localhost:8080
HTTP/1.1 200 OK
Host: localhost:8080
Date: Thu, 05 Oct 2023 07:45:57 GMT
Connection: close
X-Powered-By: PHP/8.2.10
Date: Mon, 25 Mar 1985 00:20:00 GMT
Content-type: text/html; charset=UTF-8

I have added a check condition before append the header, and a test file.

[nielsdos]

Thanks for the patch @coppolafab . This mostly looks good, I just have some nits and a remark about the case sensitivity.

[nielsdos]

Looks almost perfect, just some remark

[nielsdos]

LGTM, thanks!
I'll let the CI finish before merging, and I'll merge tomorrow or so.
EDIT: Windows CI failure is spurious

[nielsdos]

Merged manually in 8.1 and above, thanks.