Fix srcset URLs at WP Engine

fix_srcset_urls.php

<?php
/**
 * Change srcset URLs to https if the page is loaded via https
 * Activate with:
 *
 *    add_filter( 'wp_calculate_image_srcset', 'zbs_fix_srcset_for_https' );
 *
 * @param  array $sources array of sources used in srcset.
 * @return array
 */
function zbs_fix_srcset_for_https( $sources ) {
    if ( is_ssl() ) {
        foreach ( $sources as &$source ) {
            $source['url'] = set_url_scheme( $source['url'], 'https' );
        }
    }

    return $sources;
}

Based on the WP Plugin from @salcode here, but I just wanted a filter