fix switch of en/decode in mdn, thanks Masato HIGASHIYAMA (#1694687)

src/read_body.php

@@ -142,7 +142,7 @@ function SendMDN ( $mailbox, $passed_id, $message, $imapConnection) {
     }
     $rfc822_header->content_type = $content_type;
     $rfc822_header->to[] = $header->dnt;
-    $rfc822_header->subject = _("Read:") . ' ' . encodeHeader($header->subject);
+    $rfc822_header->subject = _("Read:") . ' ' . decodeHeader($header->subject,true,false);
 
     $idents = get_identities();
     $needles = array();