Only set PopupRefNid and PopupRefTitle cookie when necessary.

The popups_reference module sets two cookies each time a node is created. This does not seem to scale very well.

I am working on Pressflow (a Drupal 6 distro) behind Varnish (a reverse proxy).

I created a batch api operation that was creating many nodes at a time. After so many nodes are created and so many (of the same) cookies are sent in the headers, Varnish was failing. This may have been specific to my Varnish setup; however, popups_reference need only set the cookie if popups api was used to create the node.

Thus I created a patch that only sends the cookie, if:
$_SERVER['X-Drupal-Render-Mode'] == 'json/popups'

which serves as an indication that this node is being created through popups api.