Fix PLAIN authentication for some IMAP servers (#1488674)

CHANGELOG

@@ -1,6 +1,7 @@
 CHANGELOG Roundcube Webmail
 ===========================
 
+- Fix PLAIN authentication for some IMAP servers (#1488674)
 - Fix encoding vCard file when contains PHOTO;ENCODING=b (#1488683)
 - Fix focus issue in IE when selecting message row (#1488620)
 - Remove (too big) min-width on mail screen

program/include/rcube_imap_generic.php

@@ -530,6 +530,7 @@ class rcube_imap_generic
                 }
                 else {
                     $authc = $user;
+                    $user  = '';
                 }
                 $auth_sasl = Auth_SASL::factory('digestmd5');
                 $reply = base64_encode($auth_sasl->getResponse($authc, $pass,
@@ -568,6 +569,7 @@ class rcube_imap_generic
             }
             else {
                 $authc = $user;
+                $user  = '';
             }
 
             $reply = base64_encode($user . chr(0) . $authc . chr(0) . $pass);