Google_Service_Groupssettings_Resource_Groups::get returns object with all properties null

#### Environment details

- PHP version: 7.3
- Package name and version: "google/apiclient": "^2.11"

#### Steps to reproduce

1. try to retrieve Group Settings via PHP SDK.

#### Code example

```php
$settings = (new \Google_Service_Groupssettings($this->getClient()))->groups->get('xxx@gmail.com');
var_dump($settings); // Object Google\Service\Groupssettings\Groups with all properties null.
```
I've tried to trace the code, body of HTTP response was XML, bug SDK tries to parse the body with json_decode().
Add optParams to specify response format to JSON could solve this problem,
```php
$settings = (new \Google_Service_Groupssettings($this->getClient()))->groups->get('xxx@gmail.com', ['alt' => 'json']);
var_dump($settings); // seems ok
```

It's better for SDK to add this optParams automatically, instead of letting developer to add this param manually.

Thanks!