add clearer AddJsonBody & AddXmlBody methods

It can be unintuitive that you need to first specify the RequestFormat
is JSON to have the RestRequest serialize data to JSON. This leads to
hard to debug HTTP responses saying that the JSON format is incorrect,
because by default the data is serialized to XML. This is further
compounded by endless suggestions on popular forums and message boards
that you manually need to set the ‘Content-Type’ header to be
‘application/json’ making this a difficult product to use out of the
box.