Custom devise sign_up: json response after sign up - json

Question

i want customize devise sign_up in order that after a json sign up post request, i get a json response.
So, in app/controllers i have created session_controller.rb:
class SessionsController < Devise::SessionsController
def create
self.resource = warden.authenticate!(auth_options)
set_flash_message(:notice, :signed_in) if is_navigational_format?
sign_in(resource_name, resource)
respond_to do |format|
format.html do
respond_with resource, :location => after_sign_in_path_for(resource)
end
format.json do
render :json => { :response => 'ok'}.to_json, :status => :ok
end
end
end
end
and in routes.rb:
devise_for(:users, :controllers => { :sessions => "sessions" })
Now i want make a json post request to sign up a user and get a json response.
I set the URL: localhost:3000/users.json
Method: Post
Header: Content-Type: application/json
Data: {"user":{"email":"test#gmail.com", "password":"password123", "password_confirmation":"password123"}}
Why i don't receive the correct json response?
I receive:
Staus:406 Not Acceptable
X-Runtime: 0.102006
Date: Tue, 02 Apr 2013 16:03:03 GMT
Server: WEBrick/1.3.1 (Ruby/1.9.3/2012-02-16)
Content-Type: application/json; charset=utf-8
Cache-Control: no-cache
Connection: Keep-Alive
Content-Length: 1
X-Request-Id: 24f5d66b1de09560b88e332e06b33c1b
X-Ua-Compatible: IE=Edge