Why Nginx redirect www to non-www urls?

Hi!
As in the title my problem is about www e non www urls. I use nginxCP, a distribution of nginx that suites cPanel and WHM.
I need to be able to surf my magento sites hosted by this server with both non-www and www urls.
Ex: when I type "www.mysite.com" I surf this site with www, when I type "mysite.com" I surf this site with www. No redirect

Is there a way to do this?

In attachment you can see my config file for nginx and a site's vhost (that ise automatically generated by nginxCP)

Thank you in advance