$.ajaxSetup({
  contentType: "application/x-www-form-urlencoded; charset=utf-8"
});
$.post("test.php", { name: "i5a6", time: "2pm" },
   function(data){
     process(data);
   }, "json");
 

$.ajax({
  url:url,
  type:"POST",
  data:data,
  contentType:"application/x-www-form-urlencoded; charset=utf-8",
  dataType:"json",
  success: function(){
    ...
  }
})
 

$.ajaxSetup({
 contentType: "application/x-www-form-urlencoded; charset=utf-8"
});