Add CallStatus to request parameters

Twilio always passes CallStatus parameter when following redirects. I added it too. Didn't find a good place to test it, I think it would involve interrogating Capybara a lot. It would also be nice to customize this option in follow_redirect e.g.
if there is a `Play` followed by a `Redirect` twilio will still follow the redirect if the person hangs up, but the CallStatus will be "completed" instead of "in-progress". Let me know if you want me to add this.