Wp_mail function: email subject with multibyte chars is not encoded properly

[jetpackpony]

Problem:
When an email with multibyte characters (like Swedish é, å, etc.) in the subject is sent from Wordpress (using wp_mail), the subject of the email at the reciever end looks like this: =?UTF-8?Q?New_site_name_Site:_J=C3=A9t_inqsfzxb_p=C3=A5_?= =?UTF-8?Q?=C3=A9?=

Reproduce:
Create the new blog with a multibyte char in the blog title and then activate it.
For example: go to /wp-content/wp-signup.php and create a new user. After that you will be asked to create a new blog. Create one and put some é and å characters in the blog title. After that you will recieve the first email (which should be ok), asking to activate the account. Activate it, then you will get the second email with the screwed subject.

Reason:
The reason is phpmailer class which double-encodes the email subject (only when sending mail using php mail() function though). The problem in these lines (wp-includes/class-phpmailer.php, taken from WP 3.2.1):
(lines 657, 663, 671, 677)
$rt = @mail($val, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
$rt = @mail($to, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
$rt = @mail($val, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
$rt = @mail($to, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header);
The subject is first encoded with EncodeHeader function, and after that encoded again by mail() function itself.

Fix suggestion:
Since mail() function does proper encoding of subject, we don't need to encode it ourselves. So those lines should be replaced by:
$rt = @mail($val, $this->Subject, $body, $header, $params);
$rt = @mail($to, $this->Subject, $body, $header, $params);
$rt = @mail($val, $this->Subject, $body, $header, $params);
$rt = @mail($to, $this->Subject, $body, $header);
I've tried this on my server, and it works well.

Conclusion:
Hope it will be fixed and included in the release soon. Although, i've found the good enough temporarily solution overloading wp_mail() with a plugin, but still would be nice to see the solution in the wordpress core.

The patch based on suggested fix