Add failing test

Author: michael-misshore

spec/internal/config/critical_path_css.yml

@@ -1,5 +1,6 @@
 defaults: &defaults
   base_url: http://0.0.0.0:9292
+  manifest_name: application
   css_path: /test.css
   routes:
     - /

spec/lib/critical_path_css/rails/config_loader_spec.rb

@@ -63,6 +63,32 @@
       end
     end
 
+    context 'when no paths are specified' do
+      let(:config_file) {
+        <<~CONFIG
+          defaults: &defaults
+            base_url: http://0.0.0.0:9292
+            manifest_name: application
+            routes:
+              - /
+
+          development:
+            <<: *defaults
+
+          test:
+            <<: *defaults
+        CONFIG
+      }
+
+      it 'sets css_path with the path' do
+        expect(subject.config['css_path']).to eq '/app/spec/internal/public/test.css'
+      end
+
+      it 'leaves css_paths empty' do
+        expect(subject.config['css_paths']).to eq []
+      end
+    end
+
     context 'when single css_path and multiple css_paths are both specified' do
       let(:config_file) {
         <<~CONFIG