setupHttpClient() now properly uses the configured private key

Unless I messed things up, current version has an (obvious?) bug at line 29, Models\Client.php:

```php
'query' => ['private_token' => 'TWr7iJtT3WkMQmZrNZzJ']
```

The private token is hardcoded. As a result, any method will fail with "401 Unauthorized" error.
I've made a simple change to fix this.