modified passing of parameters in the POST portion of the Client.request method

since oauth2 discards parameters when the content type is JSON. I had to make this change in order to do POST for the flickr API method flickr.photosets.create (which is a POST with no data and a couple of required parameters). The flickr API was complaining that  my request did not include those parameters and I found out that since the content type is JSON, when these parameters get passed to oauth2 client, they get dropped since oauth2 is only looking for parameters of application/x-www-form-urlencoded.