Missing decoding of email subjects

[svandragt]

Ref: https://github.com/humanmade/Workflows/blob/75c19b91612b3ba6848b0c21b839919968181b95/lib/destinations/email.php#L74+L82= Lines 74 and 82.

If a post title contains HTML entities such as em-dash, the email destination sends emails with HTML encoded characters in the email subject. In other places in WordPress, the message subject is typically decoded using wp_specialchars_decode(), such as the following code fragment:

wp_mail( $user->user_email, wp_specialchars_decode( $subject ), $message, $message_headers );