Add rack-client as a test dependency.
Add a very basic RackClient adapter.
Add tests for the RackClient adapter.
Fix the faraday env <=> rack env conversions.
Fix request body reading in the RackClient adapter.
Add request timeouts in the RackClient adapter.
Convert Rack response body to a string in the RackClient adapter.

The Rack Spec requires the response body to respond to each and yeild
string objects.  Faraday seems to want the body to be a string, so this
builds a string body from the yeilded strings.