Add JSON response support for notifications/tray.

app/views/notifications/trays/show.json.jbuilder

@@ -0,0 +1 @@
+json.array! @notifications, partial: "notifications/notification", as: :notification

test/controllers/notifications/trays_controller_test.rb

@@ -11,4 +11,13 @@ class Notifications::TraysControllerTest < ActionDispatch::IntegrationTest
     assert_response :success
     assert_select "div", text: /Layout is broken/
   end
+
+  test "show as JSON" do
+    expected_ids = users(:kevin).notifications.unread.ordered.limit(100).pluck(:id)
+
+    get tray_notifications_path(format: :json)
+
+    assert_response :success
+    assert_equal expected_ids, @response.parsed_body.map { |notification| notification["id"] }
+  end
 end