Before this change attributes listed with @SessionAttributes would not
be saved in the session when there was a redirect and the controller
method declared a parameter of type RedirectAttributes.
This change ensures it's the "default" model that is always the one
checked for @SessionAttributes under all circumstances since
RedirectAttributes is really only meant to provide String values to
insert into or append to the the redirect URL.
Issue: SPR-12542
ea05e0b1ad