Make request body configurable for service implementors and accessible from outside

(This pull request is submitted under MIT/X11)

We're using this to implement Facebook batch requests.
This is roughly how we use it:

        public Request CreateBatchRequest (params Request [] requests)
        {
            var session = EnsureLoggedIn ();
            JArray jsonRequests = new JArray ();

            foreach (var request in requests) {
                var jsonRequest = new JObject {
                    {"method", request.Method},
                    {"relative_url", BaseUri.MakeRelativeUri (request.Url).ToString ()}
                };

                var body = request.GetRawBody (); // HERE
                if (body.Length > 0) 
                    jsonRequest ["body"] = body;

                jsonRequests.Add (jsonRequest);
            }

            var batchRequest = session.Service.CreateRequest ("POST", BaseUri, session.Account);
            batchRequest.Body = "batch=" + jsonRequests.ToString (); // AND HERE

            return batchRequest;
        }

Docs on batch requests:

https://developers.facebook.com/docs/reference/api/batch/

We didn't want to subclass `Request` and opted for keeping `Body` public because we want to make it **implementation-independent**, i.e. both `FacebookService` and `Facebook6Service` (that uses iOS 6 Social.framework) can be used with this code.