title: rails 3 namespace resources routing exception
body: <p>I'm building JSON api controllers.</p>
  
  <p>my routes.rb has:</p>
  
  <pre><code>namespace :api do
    resources :users
  end
  </code></pre>
  
  <p>controllers/api/users.rb:</p>
  
  <pre><code>respond_to :json
  def create
    @user = User.create(params[:user])
    respond_with(@user)
  end
  </code></pre>
  
  <p>when posting to api/users.json a new user is created but I'm getting an exception saying user_url method is missing.
  if i add: resources :users to routes.rb everything is fine.
  what's happening? any other way to solve this ?</p>