Expose the response string in ResourceResponse

Hi,

Currently there is no way to get the original response string for a `ResourceResponse` - there is only access to the already parsed JSON property bag.

I need to convert the response to custom class (protobuff in my case) and having to serialize using toString() is a waste since the SDK already has access to the JSON string.

It would be great if the SDK could expose this since `DocumentServiceResponse` already has a `getReponseBodyAsString`.