Problem with HttpCache and cacheControlHeader

I added to siteController’s behaviors this config




'http_cache' =>   [

                'class'                 => HttpCache::className(),

                'only'                  => [ 'index '],

                'cacheControlHeader'    => 'public, max-age=30',

            ],



It’s only a test: I expect to see the header




Cache-Control: public, max-age=30



So the browser should reload the index page from server only once every 30 secs.

Actually I’m watching the response header with firebug and this header is valorized as




Cache-Control	

no-store, no-cache, must-revalidate, post-check=0, pre-check=0



What am I doing wrong?