Ensure compatibility between ActionDispatch::Request::Session and Rack

Adding the `each` method is required for ensuring compatibility between
Rails, and other Rack frameworks (like Sinatra, etc.), that are mounted
within Rails, and wish to use its session tooling. Prior to this, there
was an inconsistency between ActionDispatch::Request::Session and
Rack::Session::Cookie, due to the absence of the `each` method. This
should hopefully fix that error. :)

For a full integration test with Sinatra and a standalone Rack
application, you can check out the gist for that here: https://gist.github.com/maclover7/08cd95b0bfe259465314311941326470.

Solves #15843.

actionpack/lib/action_dispatch/request/session.rb

@@ -9,7 +9,7 @@ class Session # :nodoc:
 
       # Singleton object used to determine if an optional param wasn't specified
       Unspecified = Object.new
-      
+
       # Creates a session hash, merging the properties of the previous session if any
       def self.create(store, req, default_options)
         session_was = find req
@@ -198,6 +198,10 @@ def merge!(other)
         @delegate.merge!(other)
       end
 
+      def each(&block)
+        to_hash.each(&block)
+      end
+
       private
 
       def load_for_read!

actionpack/test/dispatch/request/session_test.rb

@@ -114,5 +114,31 @@ def delete_session(env, id, options); 123; end
         }.new
       end
     end
+
+    class SessionIntegrationTest < ActionDispatch::IntegrationTest
+      class MySessionApp
+        def call(env)
+          request = Rack::Request.new(env)
+          request.session['hello'] = 'Hello from MySessionApp!'
+          [ 200, {}, ['Hello from MySessionApp!'] ]
+        end
+      end
+
+      Router = ActionDispatch::Routing::RouteSet.new
+      Router.draw do
+        get '/mysessionapp' => MySessionApp.new
+      end
+
+      def app
+        @app ||= RoutedRackApp.new(Router)
+      end
+
+      def test_session_follows_rack_api_contract_1
+        get '/mysessionapp'
+        assert_response :ok
+        assert_equal 'Hello from MySessionApp!', @response.body
+        assert_equal 'Hello from MySessionApp!', session['hello']
+      end
+    end
   end
 end