Ensure asset-manager redirects are cached

At the moment these redirects aren't cached which means that they won't
be cached by Fastly and will always hit our origin. This PR should
prevent this from happening and instead they'll be cached by Fastly for
1 day.

app/controllers/asset_manager_redirect_controller.rb

@@ -5,6 +5,7 @@ def show
       asset_url = Plek.new.external_url_for("draft-assets")
     end
 
+    expires_in 1.day, public: true
     redirect_to host: URI.parse(asset_url).host, status: :moved_permanently
   end
 end

test/controllers/asset_manager_redirect_controller_test.rb

@@ -6,6 +6,13 @@ class AssetManagerRedirectControllerTest < ActionController::TestCase
     Plek.any_instance.stubs(:external_url_for).returns("http://draft-asset-host.com")
   end
 
+  test "sets the cache-control max-age to 1 day" do
+    request.host = "some-host.com"
+    get :show, params: { path: "asset.txt" }
+
+    assert_equal @response.headers["Cache-Control"], "max-age=86400, public"
+  end
+
   test "redirects asset requests made via public host to the public asset host" do
     request.host = "some-host.com"
     get :show, params: { path: "asset.txt" }