federate raw versions of roars using `source` prop

app/serializers/activitypub/note_serializer.rb

@@ -8,7 +8,7 @@ class ActivityPub::NoteSerializer < ActivityPub::Serializer
              :in_reply_to, :published, :url,
              :attributed_to, :to, :cc, :sensitive,
              :atom_uri, :in_reply_to_atom_uri,
-             :conversation
+             :conversation, :source
 
   attribute :content
   attribute :content_map, if: :language?
@@ -40,6 +40,10 @@ class ActivityPub::NoteSerializer < ActivityPub::Serializer
     Formatter.instance.format(object)
   end
 
+  def source
+    { 'source' => object.proper.text, 'mediaType' => 'text/plain' }
+  end
+
   def content_map
     { object.language => Formatter.instance.format(object) }
   end