ajax problem

[boyley]

HI
code is as follows:
$.ajax({
type:'POST',
url: url,
beforeSend: function (xhrObj) {
xhrObj.setRequestHeader('Content-Type', 'application/json');
},
data:{},
success:function(data){},
error:function(data){},
});
I set request header Content-Type is application/json.But the actual request message header are as follows,: why is appended to the default message on the head:
Content-Type:application/x-www-form-urlencoded, application/json
i want remove application/x-www-form-urlencoded Media type ,leaving only application/json