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


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

What am I doing wrong?