Checks for valid HTTP Basic Auth authentication on the given request.
scope HTTPServerRequest req,
scope @safe bool delegate(string, string) pwcheck
Upon successful authorization, the name of the authorized user will
be stored in
|Request object that is to be checked
|A delegate queried for validating user/password pairs
true iff a valid Basic Auth header is present
and the credentials were verified successfully by the validation
if the "Authorization" header is malformed.