Allow hook_mail_alter to cancel mail

I use the following patch to be able to disable Drupal from sending out mail: https://drupal.org/files/hook_mail_alter-cancel-800434-90.patch (from this issue https://drupal.org/node/800434).

However, the Messaging module bypasses drupal_mail() and calls drupal_mail_send() directly, therefore bypassing any code that specifies that mail shouldn't be sent. I suggest a small patch to check for $mail['send'] before actually sending the mail. Patch to follow.