Render system: #cache processing provides no means of invalidating a cached element

The idea of the #cache property of a render array is that it will manage caching of an element. It has an 'expire' member, which you would think would mean something. (My assumption is that someone using #cache and setting '#cache['expire'] would want it to expire. However, there's no provision for that in the code.

The attached one-liner adds that in.

Example: Without this patch the following would cache the element, but it would never expire:

  $interval = 10;
  $render_array = array( t('cache demonstration') => array(
      '#markup' => t('The current time was %time when this was cached. Updated every %interval seconds', array('%time' => date('r'), '%interval' => $interval)),
      '#cache' => array(
        'keys' => array('render_example', 'cache', 'demonstration'),
        'bin' => 'cache_page',  // Not probably the best place for this.
        'expire' => time() + $interval,
        'granularity' => DRUPAL_CACHE_PER_PAGE | DRUPAL_CACHE_PER_ROLE,
      ),
      ),
  );