DoJo Toolkit additional headers not set on request

I'm using DoJo 1.6.1 for accessing my REST web service. To get the response in JSON I need to set the Aceept header field to application/json.

According to the DoJo documentation this is done by setting the headers property on the request, like to following:

var xhrArgs = {
    url: "http://localhost/myservice",
    he开发者_开发百科aders: { "Accept": "application/json" },
    handleAs: "json",
    load: successCallback,
    error: errorCallback
}

var deferred = dojo.xhrGet(xhrArgs);

I also used dojo.io.script.get(jsonpArgs); with the same issue: accept is of the value */* on the server side. I'm using Firefox 5.0.1 (also tried with Safari and Firefox on Windows).

Any suggestions what the problem is? Thanks in advance! :)