Hello,

I just finished upgrading from Symfony 2.8 to 3.4.6, and my http cache is broken when using AppCache (without debug).

It seems that since this PR, when the session is started, the response cache is set to private, which is fine.

But now when I go to a page where session is started and where I do render_esi(controller('MyBundle:Controller:action')), http cache is not working as before, because subrequests responses are made private even if I don't start / use the session.