Warning messages when sending notification emails

Whenever notification emails are sent to the email address of the newly created account, a warning message is generated because an argument passed to drupal_mail() is not an array. The $headers argument is passed as a string (in RFC header format) whereas drupal_mail() expects an associative array which it later converts to RFC format before sending the message. I have attached a patch to fix the problem.

Thanks for the module by the way -- it's exactly what I've been looking for a while. I look forward to it being officially released.