ostatus: we need to include the original author of a message as a mention

this is due to the use of activity.recipients which may not necessarily contain the original
author if the actors are not following each other.

lib/pleroma/web/ostatus/activity_representer.ex

@@ -184,7 +184,10 @@ def to_simple_form(%{data: %{"type" => "Announce"}} = activity, user, with_autho
 
     retweeted_xml = to_simple_form(retweeted_activity, retweeted_user, true)
 
-    mentions = activity.recipients |> get_mentions
+    mentions =
+      ([retweeted_user.ap_id] ++ activity.recipients)
+      |> Enum.uniq()
+      |> get_mentions()
 
     [
       {:"activity:object-type", ['http://activitystrea.ms/schema/1.0/activity']},