point to attributes if no pointer is passed

Author: JoeWoodward

lib/jsonapi/rails/serializable_active_model_errors.rb

@@ -11,7 +11,7 @@ class SerializableActiveModelError < Serializable::Error
       end
 
       source do
-        pointer @pointer unless @pointer.nil?
+        pointer(@pointer || '/data/attributes')
       end
     end