Fix broken trusted comments functionality.

This fix is slightly hackish, as we simply treat comments as whitespace.
This should largely be correct, and breaks no current test cases,
although it could result in noncompliant behavior.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

library/HTMLPurifier/Token/Comment.php

@@ -6,6 +6,7 @@
 class HTMLPurifier_Token_Comment extends HTMLPurifier_Token
 {
     public $data; /**< Character data within comment. */
+    public $is_whitespace = true;
     /**
      * Transparent constructor.
      *