Use Rack::QueryParser with nil missing value.

actionpack/lib/action_dispatch/http/request.rb

@@ -391,6 +391,20 @@ def session_options=(options)
       Session::Options.set self, options
     end
 
+    if Rack.release >= "3"
+      class QueryParser < ::Rack::QueryParser
+        def missing_value
+          nil
+        end
+      end
+
+      QUERY_PARSER = QueryParser.make_default(32)
+
+      def query_parser
+        QUERY_PARSER
+      end
+    end
+
     # Override Rack's GET method to support indifferent access.
     def GET
       fetch_header("action_dispatch.request.query_parameters") do |k|

actionpack/test/controller/test_case_test.rb

@@ -261,7 +261,7 @@ def to_param
   def test_body_stream
     params = Hash[:page, { name: "page name" }, "some key", 123]
 
-    post :render_body, params: params.dup
+    post :render_body, body: params.to_query
 
     assert_equal params.to_query, @response.body
   end

actionpack/test/dispatch/request/query_string_parsing_test.rb

@@ -168,7 +168,7 @@ def assert_parses(expected, actual)
           middleware.use(EarlyParse)
         end
 
-        get "/parse", params: actual
+        get "/parse?#{actual}"
         assert_response :ok
         assert_equal(expected, ::QueryStringParsingTest::TestController.last_query_parameters)
       end