feed link is not added to head

howdy!

when crss should be included in a page, it calls drupal_add_feed() in a preprocess_page function.

drupal_add_feed tries to add a feed link to the head of the page, so that feed readers etc. can discover the feed:

<link rel="alternate" type="application/rss+xml" title="localhost - All comments" href="http://localhost/drupal-6/crss" />

however, because $variables['head'] has already been computed at this point, the link is not getting added to head.

the solution to this is to reset $variables['head'] in commentrss_preprocess_page(), and that's what this patch does.