Fix use with kwargs

Rails also has similar issue and is already fixed in
rails/rails#38091.

lib/rack/builder.rb

@@ -101,6 +101,7 @@ def use(middleware, *args, &block)
       end
       @use << proc { |app| middleware.new(app, *args, &block) }
     end
+    ruby2_keywords(:use) if respond_to?(:ruby2_keywords, true)
 
     # Takes an argument that is an object that responds to #call and returns a Rack response.
     # The simplest form of this is a lambda object:

test/spec_builder.rb

@@ -8,7 +8,7 @@
 require 'rack/urlmap'
 
 class NothingMiddleware
-  def initialize(app)
+  def initialize(app, **)
     @app = app
   end
   def call(env)
@@ -45,7 +45,7 @@ def builder_to_app(&block)
 
   it "doesn't dupe env even when mapping" do
     app = builder_to_app do
-      use NothingMiddleware
+      use NothingMiddleware, noop: :noop
       map '/' do |outer_env|
         run lambda { |inner_env|
           inner_env['new_key'] = 'new_value'