No basic auth on redirect

- Fixes a bug where nil (empty) Basic Auth is added to a request after
  redirect
- Fixes the other side of the equation where `user` and `password` keys were sent to `Excon::Connection` but their values were `nil`, we should not add empty Authorization header in that case.
- ~~Introduces Rspec tests 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.~~
- Note that the Shindo tests did not fail for these changes, and that points me towards needing this finer-grained class testing to make sure we're handling edge cases for internal behavior correctly (things that can't be detected by what's in the response)