Fixed "Undefined property: stdClass::$status" in _processResponse()

I'm trying to update the Front of the queue with the following code:

````php
$frontCall = $this->getTwilio()->account->queues->get($queueSid)->members->front();
$frontCall->update(array(
    'Url'    => '/answer.xml'
    'Method' => 'POST'
));
````

Works fine, if the queue is not empty. But when there's no one in the queue, I get this error:

````text
Notice: Undefined property: stdClass::$status in /Volumes/www/tolkenselect/vendor/twilio/sdk/Services/Twilio.php line 216
````

This PR fixes this.