fix_sitemap.php

<?php
function fix_wp_sitemap_whitespace($input) {

    $allowed = false;
    $found = false;

    foreach (headers_list() as $header) {

        if (preg_match("/^content-type:\s+(text\/(html|application\/(xhtml\+xml|atom\+xml|xml)))/i", $header)) {
            $allowed = true;
        }

        if (preg_match("/^content-type:\s+/i", $header)) {
            $found = true;
        }
    }

    if ($allowed || $found) {
        return preg_replace("/^\s*/m", "", $input);
    } else {
        return $input;
    }
}

ob_start("fix_wp_sitemap_whitespace");
?>