Specify contentType in ajax POST request.

Set content type to 'application/json' when POSTing JSON data to the server.

This fixes a bug where any occurence of the '??' string in POSTeda answer data
would get replaced with a random jsonp callback name generated by jQuery.

If content type is set to something other than the default www-form-urlencoded,
jQuery will not mess with the data.

cf. http://bugs.jquery.com/ticket/8417