fix #656178: dc_date of a blog entry should consider creation_date to behave...

fix #656178: dc_date of a blog entry should consider creation_date to behave nicely with rss aggregators

entities.py

@@ -51,6 +51,10 @@
     def dc_description(self, format='text/plain'):
         return self.printable_value('content', format=format)
 
+    def dc_date(self, date_format=None):# XXX default to ISO 8601 ?
+        """return latest modification date of this entity"""
+        return self._cw.format_date(self.creation_date, date_format=date_format)
+
     def parent(self):
         return self.entry_of and self.entry_of[0] or None