Merge branch 'dt/http-empty-auth' into maint

http.emptyauth configuration is a way to allow an empty username to pass when attempting to authenticate using mechanisms like Kerberos. We took an unspecified (NULL) username and sent ":" (i.e. no username, no password) to CURLOPT_USERPWD, but did not do the same when the username is explicitly set to an empty string. * dt/http-empty-auth: http: http.emptyauth should allow empty (not just NULL) usernames