wp_mail miss send Content-Type with boundary when array header does not have other custom headers

[kn007]

It's a PR send in github. ​https://github.com/WordPress/WordPress/pull/273

Fix empty charset in line 436.
Before:
Content-Type: multipart/alternative; charset=
After:
Content-Type: multipart/alternative; charset=UTF-8

---

fix $headers[] = "Content-Type: multipart/alternative; boundary=\"$boundary\""; not send when not other headers.

Test Code:

<?php
$boundary = md5( uniqid() . microtime() );
$to = 'wordpress@wordpress.org';
$subject = 'TEST';
$headers[] = "From: Wordpress <wordpress@wordpress.org>";
$headers[] = "Content-Type: multipart/alternative; boundary=\"$boundary\"";
$body  = "--$boundary\r\n" .
         "Content-Type: text/plain; charset=UTF-8\r\n";
$body .= 'test';
$body .= "--$boundary\r\n" .
         "Content-Type: text/html; charset=UTF-8\r\n";
$body .= '<div>test</div>';
wp_mail( $to, $subject, $body, $headers );

Before:

After:

I could search some issue for this, like
​http://wordpress.stackexchange.com/questions/191923/sending-multipart-text-html-emails-via-wp-mail-will-likely-get-your-domain-b
​https://wordpress.org/support/topic/using-wp_mail-with-php-generated-attachments/

but still have same problem here.