Add OS detail to Nginx header

I am using nginx (1.6.2) over FreeBSD. But Nginx header is not emitting the name of my Operating System.
I checked it here: http://serverheader.com/header/form.html
[quote]
HTTP/1.1 200 OK
Server: nginx/1.6.2
Date: Thu, 23 Oct 2014 17:10:35 GMT
Content-Type: text/html; charset=UTF-8
Transfer-Encoding: chunked
Connection: keep-alive
Vary: Accept-Encoding
X-Powered-By: PHP/5.5.18
Set-Cookie: PHPSESSID=767bce909a8b2b53b25a5a6dcaf7119a; path=/
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
X-Pingback: http://vyalmicro.com/xmlrpc.php
Link: ; rel=shortlink
[/quote]

Why name of my OS is not comming and how to add it? Note my server_tokens isn't off....