Make serialization more straightforward

[jrwdexter]

It is not clear when using SetBody() that serialization instantly occurs, or how it occurs.

• [ ] Add functionality to make serialization settings more straightforward. This could be SetBody(object graph, INapSerializer serializer) or other option.
• [ ] Pull serialization out from SetBody() into Execute() methods, to delay serialization until more options could be set on the request.