Mocking Guzzle calls with phpspec in symfony

namespace AppBundle\Service;

use GuzzleHttp\ClientInterface;
use GuzzleHttp\Exception\ClientException;
use GuzzleHttp\Psr7\Response;
use GuzzleHttp\Psr7\Stream;
use Symfony\Component\HttpFoundation\Request;

class PostcodeService
{
    private $client;
    private $apiUri;

    public function __construct(
        ClientInterface $client,
        $apiUri
    ) {
        $this->client = $client;
        $this->apiUri = $apiUri;
    }

    public function get($postcode)
    {
        return $this->getDetails($postcode);
    }

    private function getDetails($postcode)
    {
        $details = null;

        try {
            /** @var Response $response */
            $response = $this->client->request(Request::METHOD_GET, $this->apiUri.$postcode);
            /** @var Stream $body */
            $body = $response->getBody();
            $details = $body->getContents();
        } catch (ClientException $e) {
        }

        return $details;
    }
}

services:
    app.service.postcode:
        class: AppBundle\Service\PostcodeService
        arguments:
            - "@app.util.guzzle"
            - "http://api.postcodes.io/postcodes/" # This would normally be in parameters.yml

    app.util.guzzle:
        class: GuzzleHttp\Client

namespace spec\AppBundle\Service;

use GuzzleHttp\ClientInterface;
use GuzzleHttp\Psr7\Response;
use GuzzleHttp\Psr7\Stream;
use PhpSpec\ObjectBehavior;
use Symfony\Component\HttpFoundation\Request;

class PostcodeServiceSpec extends ObjectBehavior
{
    // Real one: http://api.postcodes.io/postcodes/
    private $apiUri = 'http://mock.postcodes.io/postcodes/';

    function let(
        ClientInterface $client
    ) {
        $this->beConstructedWith(
            $client,
            $this->apiUri
        );
    }

    function it_should_return_empty_details_for_non_existent_postcode(
        ClientInterface $client,
        Response $response,
        Stream $body
    ) {
        $postcode = 'NON-EXISTENT';
        $result = null;

        $client->request(Request::METHOD_GET, $this->apiUri.$postcode)->shouldBeCalled()->willReturn($response);
        $response->getBody()->shouldBeCalled()->willReturn($body);
        $body->getContents()->shouldBeCalled()->willReturn($result);

        $this->get($postcode)->shouldReturn($result);
    }

    function it_should_return_actual_details_for_existent_postcode(
        ClientInterface $client,
        Response $response,
        Stream $body
    ) {
        $postcode = 'EXISTENT';
        $result = '{"status":200,"result":{"postcode":"'.$postcode.'"}}';

        $client->request(Request::METHOD_GET, $this->apiUri.$postcode)->shouldBeCalled()->willReturn($response);
        $response->getBody()->shouldBeCalled()->willReturn($body);
        $body->getContents()->shouldBeCalled()->willReturn($result);

        $this->get($postcode)->shouldReturn($result);
    }
}