Fix JSON fragments caching.

lib/jsonapi/rails/railtie.rb

@@ -54,7 +54,7 @@ def register_renderers
               ActiveSupport::Notifications.instrument('render.jsonapi-rails',
                                                       resources: resources,
                                                       options: options) do
-                renderer.render(resources, options, self).to_json
+                JSON.generate(renderer.render(resources, options, self))
               end
             end
           end

spec/render_jsonapi_spec.rb

@@ -17,4 +17,38 @@ def index
       expect(subject.key?('data')).to be true
     end
   end
+
+  context 'when using a cache' do
+    controller do
+      def serializer
+        Class.new(JSONAPI::Serializable::Resource) do
+          type 'users'
+          attribute :name
+
+          def jsonapi_cache_key(*)
+            'foo'
+          end
+        end
+      end
+
+      def index
+        user = OpenStruct.new(id: 1, name: 'Lucas')
+
+        render jsonapi: user,
+               class: { OpenStruct: serializer },
+               cache: Rails.cache
+      end
+    end
+
+    subject { JSON.parse(response.body) }
+
+    it 'renders a JSON API success document' do
+      get :index
+      expect(Rails.cache.exist?('foo')).to be true
+      get :index
+
+      expect(response.content_type).to eq('application/vnd.api+json')
+      expect(subject.key?('data')).to be true
+    end
+  end
 end