No Authorization header if username is null

CouchDb behaviour seems have changed at 1.3.1, and Basic Auth requests with empty username and password are now denied when requesting database information e.g. http://host:port/database/. This change removes the Authorization header if the username is null, preventing this error.

An alternative to this approach would be to create a new AuthenticationType.None enum element, but I'm afraid I went for the quicker change.