Headers overwritten

htmlmail_mail_alter() obliterates existing headers. This means that, in my environment, the email isn't sent because From: header is being removed. Suggest changing this code:

  $message['headers'] = array(
    'Content-Type' => 'text/html; charset=UTF-8;',
  );

to:

	if (!isset($message['headers'])) {
		$message['headers'] = array();
	}
	
	$message['headers']['Content-Type'] = 'text/html; charset=UTF-8;';