Fix the bug that serializer could not be found if the association's serializer is namespaced but the model is not

[fix #2295]

lib/active_model/serializer/lazy_association.rb

@@ -76,6 +76,7 @@ module ActiveModel
         serializer_options[:serializer_context_class] = association_options.fetch(:parent_serializer).class
         serializer = reflection_options.fetch(:serializer, nil)
         serializer_options[:serializer] = serializer if serializer
+        serializer_options[:namespace]  = reflection_options[:namespace] if reflection_options[:namespace]
         serializer_class.new(object, serializer_options)
       end