Show <activity:subject> in Atom group notice feed

Version 0.9.6 and below of StatusNet assume anything in <author> is a
Person. So, we include an <activity:subject> element, which will be
checked first by those versions of the code, only for group feeds.

At some point we'll take this out, but it's useful for now.

lib/atomgroupnoticefeed.php

@@ -85,9 +85,14 @@ class AtomGroupNoticeFeed extends AtomNoticeFeed
         $this->setId($self);
         $this->setSelfLink($self);
 
+        // For groups, we generate an author _AND_ an <activity:subject>
+        // Versions of StatusNet under 0.9.7 treat <author> as a person
+        // XXX: remove this workaround in future versions
+
         $ao = ActivityObject::fromGroup($group);
 
-        $this->addAuthorRaw($ao->asString('author'));
+        $this->addAuthorRaw($ao->asString('author').
+                            $ao->asString('activity:subject'));
 
         $this->addLink($group->homeUrl());
     }