Active Model Serializer 10 caching. It doesn't seem to be working. Why?

My toy app has a couple of models, rental_units and users. You can find the repo here

I am running Rails 5 and AMS 10.

active_model_serializers (0.10.0.rc4)
...
rails (5.0.0.beta3)
      actioncable (= 5.0.0.beta3)
      actionmailer (= 5.0.0.beta3)
      actionpack (= 5.0.0.beta3)
...

I have a RentalUnitSerializer that looks like this:

class RentalUnitSerializer < ActiveModel::Serializer
  cache key: 'rental_unit', expires_in: 3.hours
  attributes :id, :rooms, :bathrooms, :price, :price_cents

  belongs_to :user
end

This is my UserSerializer:

class UserSerializer < ActiveModel::Serializer
  cache key: 'user'
  attributes :id, :name, :email
  has_many :rental_units

  def name
    names = object.name.split(" ")
    "#{names[0].first}. #{names[1]}"
  end
end

This is part of my Gemfile:

    source 'https://rubygems.org'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '>= 5.0.0.beta3', '< 5.1'
# Use sqlite3 as the database for Active Record
gem 'sqlite3'
# Use Puma as the app server
gem 'puma'
gem 'active_model_serializers', '~> 0.10.0.rc1'
gem "dalli"
gem "memcachier"

And this is my config/environments/development.rb file:

Rails.application.configure do
  # Settings specified here will take precedence over those in config/application.rb.

  # In the development environment your application's code is reloaded on
  # every request. This slows down response time but is perfect for development
  # since you don't have to restart the web server when you make code changes.
  config.cache_classes = false

  # Do not eager load code on boot.
  config.eager_load = false

  # Show full error reports.
  config.consider_all_requests_local = true

  # Enable/disable caching. By default caching is disabled.
  if Rails.root.join('tmp/caching-dev.txt').exist?
    config.action_controller.perform_caching = true

    config.action_mailer.perform_caching = false

    config.cache_store = :memory_store
    config.public_file_server.headers = {
      'Cache-Control' => 'public, max-age=172800'
    }
  else
    config.action_controller.perform_caching = true

    config.action_mailer.perform_caching = false

    config.cache_store = :memory_store
  end

  # Don't care if the mailer can't send.
  config.action_mailer.raise_delivery_errors = false

  # Print deprecation notices to the Rails logger.
  config.active_support.deprecation = :log

  # Raise an error on page load if there are pending migrations.
  config.active_record.migration_error = :page_load


  # Raises error for missing translations
  # config.action_view.raise_on_missing_translations = true

  # Use an evented file watcher to asynchronously detect changes in source code,
  # routes, locales, etc. This feature depends on the listen gem.
  config.file_watcher = ActiveSupport::EventedFileUpdateChecker
end

Here is the strange behavior

When I visit http://localhost:3000/users, this is my logs with no sign of caching:

Started GET "/users" for ::1 at 2016-03-04 15:18:12 -0500
Processing by UsersController#index as HTML
  User Load (0.1ms)  SELECT "users".* FROM "users"
[active_model_serializers]   RentalUnit Load (0.2ms)  SELECT "rental_units".* FROM "rental_units" WHERE "rental_units"."user_id" = ?  [["user_id", 1]]
[active_model_serializers]   RentalUnit Load (0.1ms)  SELECT "rental_units".* FROM "rental_units" WHERE "rental_units"."user_id" = ?  [["user_id", 2]]
[active_model_serializers]   RentalUnit Load (0.1ms)  SELECT "rental_units".* FROM "rental_units" WHERE "rental_units"."user_id" = ?  [["user_id", 3]]
[active_model_serializers] Rendered ActiveModel::Serializer::CollectionSerializer with ActiveModel::Serializer::Adapter::JsonApi (11.0ms)
Completed 200 OK in 13ms (Views: 12.5ms | ActiveRecord: 0.5ms)

When I visit http://localhost:3000/rental_units, there is some caching?

Started GET "/rental_units" for ::1 at 2016-03-04 15:18:37 -0500
Processing by RentalUnitsController#index as HTML
  RentalUnit Load (0.4ms)  SELECT "rental_units".* FROM "rental_units"
[active_model_serializers]   User Load (0.9ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
[active_model_serializers]   CACHE (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
[active_model_serializers]   CACHE (0.0ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
[active_model_serializers]   User Load (0.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 2], ["LIMIT", 1]]
[active_model_serializers] Rendered ActiveModel::Serializer::CollectionSerializer with ActiveModel::Serializer::Adapter::JsonApi (16.1ms)
Completed 200 OK in 21ms (Views: 19.1ms | ActiveRecord: 1.4ms)

What is going on? It looks like in the latter logs, the users are being cached? I believe that since 3 different rental units belong to User 1, AMS has some default caching going on where identical SQL queries just hit a cache of some kind. So the first query for User1 hits the server's database but subsequent queries for User1 do not. Where is this cache? Is it on the server? Or some webserver?

But I think my caching strategy in my Serializer isn't working at all. Why?