Breaking changes:
- The `accesstoken::Request::authorization` method had its return type changed
  to a non-exhaustive enum.

Bug fixes:
- The `ClientMap` allows clients to choose their own ports on `localhost`
  domains, when these are registered as `IgnorePortOnLocalhost`. It previously
  failed to use the port provided in the request, using the registered constant
  instead.
- The interpretation of an authorization header has been changed. An empty
  password portion is interpreted as a public user, as by OAuth client
  implementations. These would previously be interpreted as an empty password
  instead.