In `wp_ajax_replyto_comment()`, if `$_POST['comment_type']` is set, use it for the value of `$comment_type`, which the `compact()` call has assumed is set since [8720]. It never was.

`wp_comment_reply()`'s output can be complete overridden by the `wp_comment_reply` filter, so this check is justified and makes the AJAX callback more flexible.

Props nerrad.
Fixes #29704.

Built from https://develop.svn.wordpress.org/trunk@29758


git-svn-id: http://core.svn.wordpress.org/trunk@29530 1a063a9b-81f0-0310-95a4-ce76da25c4cd