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 
I don't really know if reading would be quicker =/ sorry but the reason I'm deleting instead of reading is to keep the very same behaviour from before, see https://github.com/rails/rails/commit/212057b912627b9c4056f911a43c83b18ae3ab34#diff-ccdef70ce0e95afc8816f47f89b56c60L512

My guess is `delete` was intentionally used to drop the `use_route` key from query parameters, later mounted from the `options` here, and that would be a reserved key or something?

> Actually, where is the :use_route key actually being set? 

We set it when calling `process` or the helpers, like get post, within the params options. I think down the stack at Journey::Formatter#generate will raise ActionController::UrlGenerationError because that value is `nil` when it shouldn't.