Minor ActivityPub JSON fixes (#4214)

- Objects must have attributedTo instead of actor
- The current attribute belongs to CollectionPage, not Collection

app/serializers/activitypub/collection_serializer.rb

@@ -6,8 +6,7 @@ class ActivityPub::CollectionSerializer < ActiveModel::Serializer
     super
   end
 
-  attributes :id, :type, :total_items,
-             :current
+  attributes :id, :type, :total_items
 
   has_many :items, key: :ordered_items