add an extra format token to the primary data string so that JRuby doesn't break

Author: NullVoxPopuli

lib/active_model_serializers/json_pointer.rb

@@ -4,7 +4,7 @@ module JsonPointer
 
     POINTERS = {
       attribute:    '/data/attributes/%s'.freeze,
-      primary_data: '/data'.freeze
+      primary_data: '/data%s'.freeze
     }.freeze
 
     def new(pointer_type, value = nil)