Add test for custom_css_exclude_list

- scenarios don't seem to reset config,
  so we also need to explicitly set this
  to nil in other tests.

spec/features/asset_helper_spec.rb

@@ -56,4 +56,21 @@
       expect(page).to have_selector('link[href^="/assets/govuk_publishing_components/components/_heading-"][rel="stylesheet"]', visible: false)
     end
   end
+
+  scenario "request only unexcluded stylesheets when custom_css_exclude_list is set" do
+    GovukPublishingComponents.configure do |config|
+      config.custom_css_exclude_list = %w[notice details]
+    end
+
+    visit "/asset_helper"
+
+    within(:xpath, "//head", visible: false) do
+      expect(page).to have_xpath("//link", visible: false, count: 2)
+
+      expect(page).to have_selector('link[href^="/assets/application-"][rel="stylesheet"]', visible: false)
+      expect(page).not_to have_selector('link[href^="/assets/govuk_publishing_components/components/_notice-"][rel="stylesheet"]', visible: false)
+      expect(page).not_to have_selector('link[href^="/assets/govuk_publishing_components/components/_details-"][rel="stylesheet"]', visible: false)
+      expect(page).to have_selector('link[href^="/assets/govuk_publishing_components/components/_title-"][rel="stylesheet"]', visible: false)
+    end
+  end
 end

spec/lib/govuk_publishing_components/app_helpers/asset_helper_spec.rb

@@ -7,6 +7,7 @@
     it "exclude stylesheets already in static when exclude_css_from_static is true and the request object is not available" do
       GovukPublishingComponents.configure do |config|
         config.exclude_css_from_static = true
+        config.custom_css_exclude_list = nil
       end
 
       add_gem_component_stylesheet("button")
@@ -35,6 +36,7 @@ def request
     it "initialize asset helper then add multiple stylesheets but exclude 'button' stylesheet since it's already in static" do
       GovukPublishingComponents.configure do |config|
         config.exclude_css_from_static = true
+        config.custom_css_exclude_list = nil
       end
 
       add_gem_component_stylesheet("accordion")