mixed content warning in https

When using wordpress in https we get a mixed content warning because of a link to
https://mysite/wp-content/plugins/wp-recaptcha/recaptcha.css

Something along the lines of the following in wp-plugin.php worked for us:

static function plugins_url() {
  $siteurl = get_option('siteurl');
  if (isset($_SERVER['HTTPS'])) {
    $siteurl = str_ireplace('https://', 'https://', $siteurl);
  }
  if (WPPlugin::determine_environment() == Environment::WordPressMU)
    return $siteurl . '/wp-content/mu-plugins';
  else
    return $siteurl . '/wp-content/plugins';
}