Fix for CakeResponse::sharable() header to include private caches

$public = true would set only "s-maxage" header which is respected by proxies but ignored by browsers. Changed to set "max-age" header which is respected by both proxies and browsers (irrespective of value of  $public).
#3858
Because from what I understand in the spec, shared caches (i.e proxies) will use the max-age value unless the s-maxage value is present.  s-maxage is only useful if you want the shared caches to use a _different_ value than user-agent/browsers caches.
ref: https://stackoverflow.com/questions/15971747/it-make-sense-to-have-max-age-and-s-maxage-in-the-http-header

If s-maxage needs to be different value we can call $this->respose->sharedMaxAge() .