Make it possible to access base URL as resource by omitting path

[thekid]

The following posts directly to /v1/users:

use webservices\rest\Endpoint;

$api= new Endpoint('https://api.example.com/v1/users');
$result= $api->resource()->post(['name' => 'Test'], 'application/json');

⚠️ This change makes it necessary to pass a trailing / to be able to use relative resources!