add support for url_options in sprockets/rails_helper

Please bear with me as this is my first pull request against rails.

I realize you may not be accepting such "feature" against 3-2-stable anymore
but I would not know how to do it at this point against master/4.0
let me know, I could adapt it for master.

also let me know if 

I also made a gem for 3.2
https://github.com/mjobin-mdsol/url_options_for_assets

the reason for this, is that I am using `url_options` for auto appending parameters to url within my application.
javascripts, stylesheets and other assets also needs to have these parameter appended, but unfortunately it does not automagically works :smile: 

there it is, my little patch.

let me know if there is a better way. I could not find anything on the internet on how to do that.

thanks