Reinstate 'alternative' auth option for now

In case I've missed this – as the docs only explicitly talk about retiring
MD5. This also keeps unit tests passing without modification as they did
not explicitly test MD5.

src/GovTalk.php

@@ -779,6 +779,7 @@ public function getSenderEmailAddress()
     public function setMessageAuthentication($messageAuthType)
     {
         switch ($messageAuthType) {
+            case 'alternative':
             case 'clear':
                 $this->messageAuthType = $messageAuthType;
                 return true;