Sent mails are missing the UTF-8 charset in the "Content-Type" header

Hello,
When I use XWiki.Mail object to send a message with http://extensions.xwiki.org/xwiki/bin/view/Extension/Mail+Sender+API

Charset in mail header missing.

Without mail template a workarround is

#set ($discard = $message.addPart("text/plain", $ContMessage, {"mimetype', "text/plain ; charset=utf-8"}))

Thxs