porting session.clear fix to master branch. [#5030 state:resolved]

Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>

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

@@ -64,6 +64,11 @@ def []=(key, value)
           super(key.to_s, value)
         end
 
+        def clear
+          load_for_write!
+          super
+        end
+
         def to_hash
           load_for_read!
           h = {}.replace(self)

actionpack/test/dispatch/session/cookie_store_test.rb

@@ -30,6 +30,11 @@ def get_session_id
       render :text => "id: #{request.session_options[:id]}"
     end
 
+    def call_session_clear
+      session.clear
+      head :ok
+    end
+
     def call_reset_session
       reset_session
       head :ok
@@ -175,6 +180,23 @@ def test_getting_from_nonexistent_session
     end
   end
 
+  def test_setting_session_value_after_session_clear
+    with_test_route_set do
+      get '/set_session_value'
+      assert_response :success
+      session_payload = response.body
+      assert_equal "_myapp_session=#{response.body}; path=/; HttpOnly",
+        headers['Set-Cookie']
+
+      get '/call_session_clear'
+      assert_response :success
+
+      get '/get_session_value'
+      assert_response :success
+      assert_equal 'foo: nil', response.body
+    end
+  end
+
   def test_persistent_session_id
     with_test_route_set do
       cookies[SessionKey] = SignedBar