Include url_helpers.  Closes gh-563.

I really wanted to add tests for checking for the actual named route helpers, but I couldn't figure it out with the way the fake-ish Rails app is set up (I'm less familiar with the routing portion of Rails than ActiveModel/AR).  I started refactoring the tests to use a `Dummy::Application` like that of Rails plugins, but a) wasn't sure if that's where you'd want this to go, and b) started getting in over my head, time-wise.  It seems like since routing helpers are now involved (which means routes), we'd be better of using a full-blown dummy app.  That would seem to make test maintenance a little more straightforward for AMS-newcomer-devs, in my opinion--especially since AMS doesn't seem to have plans for much love from the creators (not a slam--just stating what I've learned).

**TL;DR** There should be more tests here, but I think it would require lots of work just for this little change and I don't quite see the value in the trade-off at this point.