set json in guzzle http client code example

Example 1: guzzlehttp post json example

use GuzzleHttp\Client;

$client = new Client();

$response = $client->post('url', [
    GuzzleHttp\RequestOptions::JSON => ['foo' => 'bar'] // or 'json' => [...]
]);

Example 2: php convert guzzle response to json

$response = GuzzleHttp\get('http://httpbin.org/get');
if ($response->getBody()) {
    echo $response->getBody();
    // JSON string: { ... }
}