Space characters in ReturnTo query parameter cause redirect failure [#3501193]

Problem/Motivation

It seems that having a space character in the ReturnTo query parameter causes certain things to break down.

Steps to reproduce

Attempting to sign into a site via https://example.com/saml_login?ReturnTo=https://example.com/some%20path will lead to the user not being returned to /some%20path, but rather the default user profile page.

This is happening because in the following code block within \Drupal\simplesamlphp_auth\Controller\SimplesamlphpAuthController::authenticate is passing an invalid URL through to the UrlHelper.

    // See if a URL has been explicitly provided in ReturnTo. If so, use it
    // otherwise, use the HTTP_REFERER. Each must point to the site to be valid.
    $request = $this->requestStack->getCurrentRequest();

    if (($return_to = $request->query->get('ReturnTo')) ||
        ($return_to = $request->request->get('ReturnTo')) ||
        ($return_to = $request->server->get('HTTP_REFERER'))) {
      // !! $return_to is "https://example.com/some path" here where "some path" is not encoded properly!
      // This causes the externalIsLocal check to fail.
      if ($this->pathValidator->isValid($return_to) && UrlHelper::externalIsLocal($return_to, $base_url)) {
        $redirect = $return_to;
      }
    }