Use String#dup before mutating the result of Symbol#to_s

Follow up #37303.

https://buildkite.com/rails/rails/builds/63957#dc16cd12-10a2-435c-9b64-b8332114d92a/981-1293

actionpack/lib/action_controller/metal/params_wrapper.rb

@@ -115,7 +115,7 @@ def include
 
               if m.respond_to?(:nested_attributes_options) && m.nested_attributes_options.keys.any?
                 self.include += m.nested_attributes_options.keys.map do |key|
-                  key.to_s.concat("_attributes")
+                  key.to_s.dup.concat("_attributes")
                 end
               end
 

actionpack/lib/action_controller/test_case.rb

@@ -460,6 +460,7 @@ def head(action, **args)
       def process(action, method: "GET", params: nil, session: nil, body: nil, flash: {}, format: nil, xhr: false, as: nil)
         check_required_ivars
 
+        action = action.to_s.dup
         http_method = method.to_s.upcase
 
         @html_document = nil
@@ -491,11 +492,11 @@ def process(action, method: "GET", params: nil, session: nil, body: nil, flash:
           parameters[:format] = format
         end
 
-        generated_extras = @routes.generate_extras(parameters.merge(controller: controller_class_name, action: action.to_s))
+        generated_extras = @routes.generate_extras(parameters.merge(controller: controller_class_name, action: action))
         generated_path = generated_path(generated_extras)
         query_string_keys = query_parameter_names(generated_extras)
 
-        @request.assign_parameters(@routes, controller_class_name, action.to_s, parameters, generated_path, query_string_keys)
+        @request.assign_parameters(@routes, controller_class_name, action, parameters, generated_path, query_string_keys)
 
         @request.session.update(session) if session
         @request.flash.update(flash || {})