URL Parameter "accept" not working?

Hi all,

I am trying to make a API request with URL Parameters. And it works, except the accept parameter. I follow the instructions here:
https://wiki.dreamfactory.com/DreamFactory/API

My request is:
https://[username%40domain]:[password]@[serverurl]:443/api/v2/[service]/_proc/[procname]?api_key=[apikey]&accept=application%2Fxml

The result is always JSON. No error messages. It looks like the accept=application/xml URL parameter is ignored?

Or is something wrong with my request string?

Thx
Reto E.