Doesn't invalidate cache when closing comment

Issue Summary

When comment_closer closes a comment, it doesn't invalidate the page cache. The outcome of this is that the comment form will still be available, though non-functional.

Reproduction:
1) Enable caching
2) Set comment_closer to close nodes after a given period of time
3) Visit node after given period of time

Expected result:
1) The comment form shouldn't be displayed

Actual result:
1) The comment form is displayed
2) Submitting a comment simply reloads the page