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.
Fixes #21659.

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

tests/phpunit/includes/mock-mailer.php

@@ -17,7 +17,7 @@ class MockPHPMailer extends PHPMailer {
 			'to'      => $this->to,
 			'cc'      => $this->cc,
 			'bcc'     => $this->bcc,
-			'header'  => $this->MIMEHeader,
+			'header'  => $this->MIMEHeader . $this->mailHeader,
 			'subject' => $this->Subject,
 			'body'    => $this->MIMEBody,
 		);