It's based specifying application's script_name with:
Rails.application.default_url_options = {:script_name => "/foo"}
default_url_options method is delegated to routes. If router
used to generate url differs from the router passed via env
it always overwrites :script_name with this value.
This requires knowledge about original SCRIPT_NAME and
the parent router. It should be pass through the env
as ORIGIAL_SCRIPT_NAME and action_dispatch.parent_routes
This technique is here to allow using routes from Engine in Application
and vice versa. When using Engine routes inside Application it should
generate prefix based on mount point. When using Engine routes inside
Engine it should use env['SCRIPT_NAME']. In any other case it should
generate prefix as env should not be even available.
