Generate the cookies_serializer configuration with :marshal when upgr…

…ading

We were generating this configuration with :json what may break
applications upgrating from Rails 4.0-.

Now we always generate with a safer value and people is free to choose
hybrid if they want.

We don't generate with hybrid by default because there are some cases
that can fail. These cases are explained on the upgrading guide.

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -86,6 +86,13 @@ def config
       end
     end
 
+    def config_when_updating
+      foo = File.exist?('config/initializers/cookies_serializer.rb')
+      config
+
+      gsub_file 'config/initializers/cookies_serializer.rb', /json/, 'marshal' unless foo
+    end
+
     def database_yml
       template "config/databases/#{options[:database]}.yml", "config/database.yml"
     end
@@ -188,6 +195,10 @@ def create_config_files
         build(:config)
       end
 
+      def update_config_files
+        build(:config_when_updating)
+      end
+
       def create_boot_file
         template "config/boot.rb"
       end

railties/lib/rails/tasks/framework.rake

@@ -55,7 +55,7 @@ namespace :rails do
     # desc "Update config/boot.rb from your current rails install"
     task :configs do
       invoke_from_app_generator :create_boot_file
-      invoke_from_app_generator :create_config_files
+      invoke_from_app_generator :update_config_files
     end
 
     # desc "Adds new executables to the application bin/ directory"