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
Agreed. I usually use spacing liberally but the rest of the class has very little (most methods have no spacing, 2-3 methods have lots of it) so I wasn't sure what the convention was.  The original sharable() has none so I went with that convention. :)

@markstory I'll take a look at the tests