WordPress's workaround for is_ssl() function when proxying from https to http. HTTP_X_FORWARDED_PROTO feature. Stolen from https://wordpress.org/support/topic/request-modify-is_ssl-function-to-check-for-http_x_forwarded_proto

gistfile1.php

/**
 * Determine if SSL is used.
 *
 * @since 2.6.0
 *
 * @return bool True if SSL, false if not used.
 */
function is_ssl() {
  if ( isset($_SERVER['HTTPS']) ) {
    if ( 'on' == strtolower($_SERVER['HTTPS']) )
      return true;
    if ( '1' == $_SERVER['HTTPS'] )
    return true;
  } elseif ( isset($_SERVER['SERVER_PORT']) && ( '443' == $_SERVER['SERVER_PORT'] ) ) {
    return true;
  } elseif ( isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && strtolower($_SERVER['HTTP_X_FORWARDED_PROTO']) == 'https' ) {
    return true;
  }
  return false;
}