Verify external cache are working properly

https://www.drupal.org/node/2364011

(worth also checking https://www.drupal.org/node/2364011#comment-9291895)
why not use ``drupal_page_footer()``?:
1. Is there a use case for needing our own ``exit`` hook instead of the regular one?
1. Is there a use case for setting the ``cache`` variable, but not setting restful's page cache?