close the response body and constrains RoundTrip implementation to spec (return nil response when returning error)
custom errors export status code of last attempt
improved loop, removed status code from custom error, inverse branches in Attempt.StatusCode()
remove Attempt.StatusCode convenience function
Merge pull request #15 from tchakabam/close-response-and-obey-roundtripper-spec

close the response body and constrains RoundTrip implementation to spec