This fixes an regression where _routes were set incorrectly when the
inheritance chain went from one route namespace (isolated engine or main
app) to another and then back to the original. Because the url_helpers
module was being cached and was being re-included this was not setting
_routes.
This commit solves the issue by detecting that case and redefining
_routes in that case. We could have always performed the redefinition,
but it's a pretty uncommon case, so we might as well only do it when
necessary.
d6ca73306c