Changes for Faraday 0.9

## _WIP_

I wanted to take another crack at moving the project to the latest Faraday 0.9 changes. A couple of questions, @mislav:

It looks like the underlying behavior for parsing URLs with params like `:access_token => nil` has changed. That now gets parsed like `?access_token&` and the value is a `TrueClass`. I disabled the spec until we decided _how_ it should behave.

Do you mind taking a look at the changes needed for `RackCompatible`? At first glance I'm not sure if it's a mountain or a molehill.