make sure commenting works when caching with modified headers

CHANGES.txt

@@ -1,6 +1,13 @@
 Changelog
 =========
 
+2.1.3 (unreleased)
+------------------
+
+- Set modified date of object receiving comments so that caching works
+  correctly (304s)
+  [eleddy]
+
 2.1.2 (2011-12-21)
 ------------------
 

plone/app/discussion/comment.py

@@ -206,9 +206,14 @@ def notify_content_object(obj, event):
     """Tell the content object when a comment is added
     """
     content_obj = aq_parent(aq_parent(obj))
+    # set the modified date and reindex the item accordingly
+    # so that 304s work correctly. This means that adding a comment
+    # effectively counts as modifying the content type. 
+    content_obj.setModificationDate()
     content_obj.reindexObject(idxs=('total_comments',
                                     'last_comment_date',
-                                    'commentators',))
+                                    'commentators',
+                                    'modified'))
 
 
 def notify_content_object_deleted(obj, event):