Fixes #3604: default to ini setting if Cookie::$domain not available

We are using Cookie parameters for Session Native with session_set_cookie_params.

session_set_cookie_params overrides `session.cookie_domain` ini setting.

When the developer specifies `session.cookie_domain` ini setting and keeps Cookie::$domain NULL, PHP defaults to server name (which is not what the developer wants).

From PHP manual:

> Default is none at all meaning the host name of the server which generated the cookie

http://www.php.net/manual/en/session.configuration.php#ini.session.cookie-domain