Add Rack::Lint to RequestId tests

This adds additional test coverage to RequestId to validate that its
input and output follow the Rack SPEC.

In this case, the only changes necessary were to the Request tests. This
is due to the fact that the Request and Response tests use different
classes for their Response headers. The Response tests simulate a Rails
app, where the Response headers will be a Rack::Headers object for
compatbility with both Rack 2 and 3. However, since the Request tests
are only using the Hash returned by the test app, the tests must use a
downcased header to support both Rack 2 and Rack 3.

actionpack/test/dispatch/request_id_test.rb

@@ -8,7 +8,7 @@ class RequestIdTest < ActiveSupport::TestCase
   end
 
   test "passing on the request id via a configured header" do
-    assert_equal "external-uu-rid", stub_request({ "HTTP_TRACER_ID" => "external-uu-rid" }, header: "Tracer-Id").request_id
+    assert_equal "external-uu-rid", stub_request({ "HTTP_TRACER_ID" => "external-uu-rid" }, header: "tracer-id").request_id
   end
 
   test "ensure that only alphanumeric uurids are accepted" do
@@ -33,8 +33,17 @@ class RequestIdTest < ActiveSupport::TestCase
   end
 
   private
-    def stub_request(env = {}, header: "X-Request-Id")
-      ActionDispatch::RequestId.new(lambda { |environment| [ 200, environment, [] ] }, header: header).call(env)
+    def stub_request(env = {}, header: "x-request-id")
+      app = lambda { |_env| [ 200, {}, [] ] }
+      env = Rack::MockRequest.env_for("", env)
+
+      Rack::Lint.new(
+        ActionDispatch::RequestId.new(
+          Rack::Lint.new(app),
+          header: header,
+        )
+      ).call(env)
+
       ActionDispatch::Request.new(env)
     end
 end
@@ -75,7 +84,9 @@ def with_test_route_set(header: "X-Request-Id")
         end
 
         @app = self.class.build_app(set) do |middleware|
+          middleware.use Rack::Lint
           middleware.use ActionDispatch::RequestId, header: header
+          middleware.use Rack::Lint
         end
 
         yield