fix xoauth2 client mechanism

Author: grs

lib/sasl.js

@@ -144,7 +144,7 @@ XOAuth2Client.prototype.start = function(callback) {
     count += 1;
     response.writeUInt8(1, count);
     count += 1;
-    callback(response);
+    callback(undefined, response);
 };
 
 /**