Always set a Content-transfer-encoding header in MIME multiparts, fix…

…es an error flagged by IETF msglint:

ERROR: mismatch between inner content-transfer-encoding and that of enclosing multipart.

class.phpmailer.php

@@ -1501,10 +1501,10 @@ public function GetMailMIME() {
         break;
       default:
         // Catches case 'plain': and case '':
-        $result .= $this->HeaderLine('Content-Transfer-Encoding', $this->Encoding);
         $result .= $this->TextLine('Content-Type: '.$this->ContentType.'; charset='.$this->CharSet);
         break;
     }
+    $result .= $this->HeaderLine('Content-Transfer-Encoding', $this->Encoding);
 
     if($this->Mailer != 'mail') {
       $result .= $this->LE;