Fix KickBox response handling

@njam please review. The magic comes from `Kickbox\HttpClient\ResponseHandler::getBody()`:
```php
public static function getBody(GuzzleResponse $response)
{
    $body = $response->getBody(true);

    // Response body is in JSON
    if ($response->isContentType('json')) {
        $tmp = json_decode($body, true);

        if (JSON_ERROR_NONE === json_last_error()) {
            $body = $tmp;
        }
    }

    return $body;
}
```