Mail: Improve handling of UTF-8 address headers.
Previously, wp_mail() implemented Reply-To as a generic header, using
PHPMailer's addCustomHeader(). As such, the email address portion of
the header was being incorrectly encoded when the name portion
contained UTF-8 characters. Switching to PHPMailer's more specific
addReplyTo() method fixes the issue.
For greater readability, the handling of all address-related headers
(To, CC, BCC, Reply-To) has been standardized.
Props szepe.viktor, iandunn, bpetty, stephenharris.