Anchor a regular expression in wp-mail.php to ensure we are matching the...

Anchor a regular expression in wp-mail.php to ensure we are matching the Reply-To: header, not the In-Reply-To: header.

Per RFCs *822, mail headers must start at the beginning of a line. This same pattern could be continued throughout this file.

props solarissmoke.
fixes #18923.



git-svn-id: https://core.svn.wordpress.org/trunk@20682 1a063a9b-81f0-0310-95a4-ce76da25c4cd