Fix for #9058 and #8941, append relative route also in route helpers

This is a continuum of #12086, which was wrongly merged empty.

This code as is works, and prepends the relative root when *_path helper is called:

````sh
› RAILS_RELATIVE_URL_ROOT='/tributos' rails c
Loading development environment (Rails 4.2.0.beta4)
>> Rails.application.routes.url_helpers.users_path
=> "/tributos/users"
>> exit
````

Posting this for now seeking some guidance on how to better use "config.relative_url_root" (rather than using the ENV directly.
As far as I could see, using config inside RouteSet, would require a major API change. thoughts?

/cc @iainbeeston @pixeltrix