Content-type overwrite

[mrova]

Setting type like this:

$html->type = 'text/html';

in https://github.com/mtymek/MtMail/blob/master/src/Service/Composer.php#L166 causes problems with utf-8 encoding on some web mailboxes.

It should be:

$html->type = 'text/html; charset=UTF-8';

or setting it in config "default_headers" like:

'Content-Type' => 'text/html; charset=UTF-8',

should have higher priority.