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.
Fixes#21659.
git-svn-id: https://develop.svn.wordpress.org/trunk@38058 602fd350-edb4-49c9-b593-d223f7449a82
af068cdbe0