Fix "Fatal error: Cannot use object of type stdClass as array" errors

When an embed object returned which has no own GraphObject subclass the resulting GraphObject instance recieves an stdObject for backingData which is wrong since the getProperty method expects it to be an array.

Example:
    
    $me = (new FB\FacebookRequest(
        $session, 'GET', '/me?fields=email,picture'
    ))->execute()->getGraphObject(FB\GraphUser::className());
    $me->getProperty('picture')->getProperty('url'); // Fix "Fatal error: Cannot use object of type stdClass as array"
    var_dump(is_array($me->getProperty('picture')->asArray())); // prints "false"