Can I find out how to update support for 304 Not Modifield?

Question

Can I find out how to update support for 304 Not Modifield?

Does anyone know how to update support for 304 Not Modifield? Because now there will be an error to modify. I use etag to check if json should be updated or not.

@GET("/imageslist.json") Response fetch(@Header("If-None-Match") String etag);

This is how I process it now

ImageService service = restAdapter.create(ImageService.class);
Response response = null;
try {
    response = service.fetch("\"vENxxg\"");
} catch (RetrofitError e) {
    if (e.getResponse().getStatus() == HttpURLConnection.HTTP_NOT_MODIFIED) {
        //Nothing to update
        response = e.getResponse();
    } else {
        Log.e("Retrofit", e.getMessage(), e);
    }
}