Wrong HTML charset iso-8859-1 if no attachments are added

[schorschii]

I'm using the following code:

$mime = new Mail_mime("\r\n");
$mime->setHTMLBody(' öäü ');

$mail = Mail::factory('sendmail');
$mail->send($email,
	$mime->headers([
		'From' => 'test@example.com',
		'Subject' => '=?UTF-8?B?'.base64_encode("my subject").'?=',
	]),
	$mime->get([
		'text_encoding' => '7bit',
		'text_charset'  => 'UTF-8',
		'html_charset'  => 'UTF-8',
		'head_charset'  => 'UTF-8',
	])
);

But I erroneously receive an email with Content-Type: text/html; charset=ISO-8859-1 header and <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">, and therefore the special chars are broken.

The interesting part now is when adding an attachment (following code), the HTML charset is now suddenly correctly set to UTF-8 in the header and <meta> tag!

$mime->addAttachment(
	'BEGIN:VCALENDAR ...',
	'text/calendar', 'calendar-event.ics', false, 'plain'
);

Can somebody tell me if I'm doing anything wrong with the UTF-8 encoding or is this a bug?