How can I replace the 'Accept' http header in AS3 - urlrequest

Question

all
I send a http request from flash client(AS3) to a RESTFull service. The server side response json or xml data depend on the 'Accept' parameter in http header. But, I always accept xml format data even if I set the 'Accept' to 'application/json' in the client side. With wireshark I found that there are double 'Accept' parameter in the http header. Can somebody tell me why ? And/or how to get out of this.
POST /psplatform/rest/szdata/all HTTP/1.1
Host: 203.175.156.88:8080
User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:26.0) Gecko/20100101 Firefox/26.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: zh-cn,zh;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Connection: keep-alive
Content-type: application/json
Accept: application/json
Content-length: 8
public function reload():void{
data = new Object();
new URLLoader(createJSONURLRequest("http://203.175.156.88:8080/psplatform/rest/szdata/all")).addEventListener(Event.COMPLETE, loaderCompleteHandler);
}
private function createJSONURLRequest(url:String):URLRequest{
var urlRequest:URLRequest = new URLRequest(url);
urlRequest.method = URLRequestMethod.POST;
urlRequest.contentType = "application/json";
//var urlVariables:URLVariables = new URLVariables("{}");
urlRequest.data = "{name:0}";
urlRequest.requestHeaders.push(new URLRequestHeader("Accept", "application/json"));
return urlRequest;
}