XF 1.5 How to enforce all login user to have "xf_user" cookie stored on their browser?

[rdn]

I have this problem with fastcgi_cache.
I'm using this config to bypass cache for user's having this cookies:

map $http_cookie $nocachecookie {
default 0;
~xf_session_admin 1;
~xf_user 1;
~xf_fbUid 1;
}

Now, on rare cases, maybe out of 500 request 1% is still being cached, because of having no cookies stored .

I already force "Remember Always Checked" on helper_login_form and login_bar_form template with this modification:

<input type="hidden" name="remember" value="1" />

How can I force it directly on XenForo Core PHP Files?

Thanks!