Set session options id to nil is respected and cancels lazy loading.

actionpack/lib/action_dispatch/middleware/session/abstract_store.rb

@@ -24,9 +24,9 @@ module ActionDispatch
 
         def [](key)
           if key == :id
-            load_session_id! unless super(:id) || has_session_id?
+            load_session_id! unless key?(:id) || has_session_id?
           end
-          super(key)
+          super
         end
 
       private

actionpack/test/dispatch/session/cookie_store_test.rb

@@ -44,7 +44,12 @@ class CookieStoreTest < ActionController::IntegrationTest
       session[:foo] = 'bye!' * 1024
       head :ok
     end
-    
+
+    def change_session_id
+      request.session_options[:id] = nil
+      get_session_id
+    end
+
     def rescue_action(e) raise end
   end
 
@@ -212,6 +217,19 @@ class CookieStoreTest < ActionController::IntegrationTest
     end
   end
 
+  def test_setting_session_id_to_nil_is_respected
+    with_test_route_set do
+      cookies[SessionKey] = SignedBar
+
+      get "/get_session_id"
+      sid = response.body
+      assert_equal sid.size, 36
+
+      get "/change_session_id"
+      assert_not_equal sid, response.body
+    end
+  end
+
   def test_session_store_with_expire_after
     with_test_route_set(:expire_after => 5.hours) do
       # First request accesses the session