Fix version prefix in specs [WIP]

I've got a failing spec, but I'm struggling with the fix.

I can see that parameter defaults aren't getting passed through to the controller, and thus the prefix removal stuff isn't working.

I don't understand how it can actually work with Rails, but fail for a spec yet.

I've also cleaned up the rspec `have_exposed` error messages. They were a bit funky, and now they make a bit more sense.