Filter to force sitemap.xsl to load via http protocol (rather than https) for http://wordpress.org/plugins/google-sitemap-plugin/

fe-google-sitemap-plugin-https-fix.php

add_filter( 'plugins_url', 'fe_google_sitemap_plugin_https_fix', 10, 3 );

function fe_google_sitemap_plugin_https_fix( $url, $path, $plugin ) {
	if ( 'sitemap.xsl' !== $url ) {
		return $url;
	}
	if ( 'https://' === substr( $url, 0, 8 ) ) {
		// the url begins with https, which we do NOT want
		// change url to start with http instead of https
		$url = 'http' . substr( $url, 5 );
	}
	return $url;
}