AUTH PLAIN bug with qpsmtpd and alpine

Trying to get SMTP auth working with alpine, I came across a bug.
Alpine sends AUTH PLAIN and waits for a 334 response, then sends the
auth string. According to the RFC, the server should reply with 334
and a nothing else, but in Auth.pm qpsmtpd responds with "334 Please
continue."   the "Please continue" is interpreted as a non-zero
length initial challenge which causes alpine (and maybe other
clients?) to abort the session.

Signed-off-by: Charlie Brady <charlieb@budge.apana.org.au>
Signed-off-by: Robert <rspier@pobox.com>

lib/Qpsmtpd/Auth.pm

@@ -20,7 +20,7 @@ sub SASL {
 
     if ( $mechanism eq "plain" ) {
         if (!$prekey) {
-          $session->respond( 334, "Please continue" );
+          $session->respond( 334, " " );
           $prekey= <STDIN>;
         }
         ( $loginas, $user, $passClear ) = split /\x0/,