Fixed error on user page.

[danepowell]

When you visit the user page (admin/access/users) on a cold cache, you get the following error:

Warning: strpos() expects parameter 1 to be string, object given in Drupal\Component\Utility\UrlHelper::stripDangerousProtocols() (line 352 of core/lib/Drupal/Component/Utility/UrlHelper.php)

This is because Url:fromRoute returns a URL object, not a string.

[snufkin]

The change correctly uses the toString() method to cast the URL object into string, which resolves the error.