fix issue with latest version of feedgenerator

src/sigal/plugins/feeds.py

@@ -70,6 +70,7 @@ def generate_feed(gallery, medias, feed_type=None, feed_url="", nb_items=0):
         feed.add_item(
             title=Markup.escape(item.title or item.url),
             link=link,
+            content=None,
             # unique_id='tag:%s,%s:%s' % (urlparse(link).netloc,
             #                             item.date.date(),
             #                             urlparse(link).path.lstrip('/')),