use different boundary prefix FS#2846

The used boundary should have been correct but might throw off MTAs not
expecting more than two dashes at the start. This is just a wild guess
and may not fix anything (but shouldn't break anything either).

inc/Mailer.class.php

@@ -44,7 +44,7 @@ class Mailer {
         if(strpos($server,'.') === false) $server = $server.'.localhost';
 
         $this->partid   = md5(uniqid(rand(), true)).'@'.$server;
-        $this->boundary = '----------'.md5(uniqid(rand(), true));
+        $this->boundary = '__________'.md5(uniqid(rand(), true));
 
         $listid = join('.', array_reverse(explode('/', DOKU_BASE))).$server;
         $listid = strtolower(trim($listid, '.'));