Pass route_name from use_route on path_for [4.2.0 regression]

engines need it to be able to get a proper request uri on tests, the
value was lost in https://github.com/rails/rails/commit/212057b912627b9c4056f911a43c83b18ae3ab34

will look into adding a regression test

// @tenderlove 
Why do you need to delete the route name from the options hash? Surely just reading it would work and be quicker?