Problem with user picture resource into GraphObject

The end point data from user picture resource in Graph API (https://developers.facebook.com/docs/graph-api/reference/v2.0/user/picture/) is pushed into "data" key insted into root (I don't know why, is it correct?). \Facebook\GraphObject object from this response is creating wrong, property "backingData" is an instance of \stdClass insted an array and all of the methods that operate on this property as an array throw a fatal error "Cannot use object of type stdClass as array". This commit fix the issue.