No basic auth on redirect

- Fixes a bug where nil (empty) Basic Auth is added to a request after
  redirect
- Introduces an rspec test as a means of testing only the
  Excon::Middleware::RedirectFollower class and stub out other
  interacting classes -- note that minitest does not provide this kind of doubling, mocking, or stubbing to expect `Excon::Connection.new` to be called with certain params.