Flex :: Setting JSON Content Type On HTTPService?

I am trying to set the json content type on httpservice to make REST service return the json data. When I add the content type in fiddler all works fine so the problem lays in flex application, not in the web-service. But the code below does not work and I get the xml data instead of json.

mxml:
<s:HTTPService id="service" method="POST" url="server.com" result="loaded(event)" fault="fault(event)" useProxy="false" resultFormat="text">

Actionscript:
public function loadAllSamples():void {
service.contentType = "application/json";
service.send('something');
}