Eliminate the use of `extract()` in `wp_mail()`. Check the filtered array for each value before re-setting variables.

See #22400.

Built from https://develop.svn.wordpress.org/trunk@28425


git-svn-id: http://core.svn.wordpress.org/trunk@28252 1a063a9b-81f0-0310-95a4-ce76da25c4cd