Request object that is to be checked
A delegate queried for validating user/password pairs
Returns true iff a valid Basic Auth header is present and the credentials were verified successfully by the validation callback.
Throws a HTTPStatusExeption with HTTPStatusCode.badRequest if the "Authorization" header is malformed.
Checks for valid HTTP Basic Auth authentication on the given request.
Upon successful authorization, the name of the authorized user will be stored in req.username.