Add ensure block to make sure the state is properly restored

actionpack/lib/action_dispatch/middleware/show_exceptions.rb

@@ -28,7 +28,7 @@ module ActionDispatch
 
     def call(env)
       begin
-        response  = @app.call(env)
+        response = @app.call(env)
       rescue Exception => exception
         raise exception if env['action_dispatch.show_exceptions'] == false
       end

actionpack/test/controller/show_exceptions_test.rb

@@ -104,7 +104,7 @@ module ShowExceptions
       get '/', {}, 'HTTP_ACCEPT' => 'text/json'
       assert_response :internal_server_error
       assert_equal 'text/plain', response.content_type.to_s
-
+    ensure
       @app.instance_variable_set(:@exceptions_app, @exceptions_app)
       $stderr = STDERR
     end