Webform Mail UTF-8

Hello,

I tried to change the Content-Type-Header of the Mails sent out by Webform.

In the template.php of my theme I inserted:

 function phptemplate_webform_mail_headers_1926($form_values, $node, $sid, $cid) {
  $headers = array(
    'Content-Type'  => 'text/html; charset=UTF-8; format=flowed; delsp=yes',

    'X-Mailer'      => 'Drupal Webform (PHP/'. phpversion() .')',
  );
  return $headers;
}

But in the Mail Source Code there is still charset="iso-8859-1".

Is there any possibility to change this?

Regards

Adriana