remove more direct `env` mutations

actionpack/lib/action_controller/test_case.rb

@@ -98,7 +98,9 @@ def assign_parameters(routes, controller_path, action, parameters, generated_pat
         set_header 'rack.input', StringIO.new(data)
       end
 
-      @env["PATH_INFO"] ||= generated_path
+      get_header("PATH_INFO") do |k|
+        set_header k, generated_path
+      end
       path_parameters[:controller] = controller_path
       path_parameters[:action] = action
 

actionpack/test/controller/test_case_test.rb

@@ -160,7 +160,7 @@ def generate_url(opts)
   def setup
     super
     @controller = TestController.new
-    @request.env['PATH_INFO'] = nil
+    @request.delete_header 'PATH_INFO'
     @routes = ActionDispatch::Routing::RouteSet.new.tap do |r|
       r.draw do
         get ':controller(/:action(/:id))'