Allow a single instance of a Response object to be set as the next mocked response

The `GuzzleTestCase` class has a `setMockedResponse` method that allows you to do the following:

    $this->setMockedResponse($client, 'some_response')

where `some_response` is a mock response file. The `setMockedResponse` also allows you to specify response object instances, but only when added as an elements to an array:

    $this->setMockedResponse($client, array(new Guzzle\Http\Message\Response( /* ... */ )));

This PR will allow the following for consistency:

    $this->setMockedResponse($client, new Guzzle\Http\Message\Response( /* ... */ ));