Android Volley app header change application/xml to application/json

I have the following request. The API sends by default application/xml... i would like to change the header to this application/json to get JSON... how can i do that? it is not working now... Value ?xml of type java.lang.String cannot be converted to JSONObject

  JsonObjectRequest jr = new JsonObjectRequest(Request.Method.GET, url, null,        
  new Response.Listener<JSONObject>() {
        @Override
        public void onResponse(JSONObject response) {
            Log.i(TAG, response.toString());
            parseJSON(response);
            va.notifyDataSetChanged();
            Toast.makeText(getApplicationContext(), response.toString(), Toast.LENGTH_LONG).show();

            pd.dismiss();
        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            Log.i(TAG, error.getMessage());
        }
    }){
        public String getBodyContentType()
        {
            return "application/json";
        }};



    mRequestQueue.add(jr);


}