FIX Unset parent mime part body, not just children

The message itself is a MimePart, where the PGP signer will set
the contents as children (and clear existing ones), it wrongly did
not unset the body - meaning that a message as a single body was to
be transmitted as plain text as the message body, then also encrypted
as an attachemnt. This made PGP encryption pointeless, and is a rather
glaring bug.

lib/SwiftSignerCryptGPG.php

@@ -87,6 +87,7 @@ public function signMessage(Swift_Message $message)
         // Prepare message final form
         $message
             ->setEncoder($encoder)
+            ->setBody(null)
             ->setChildren([$pgpmime, $encryptedMessage]);
 
         $type = $message->getHeaders()->get('Content-Type');