Avoid loading ActionController::TestCase in production

Ref: Shopify#3

> By referencing that object, it brings in all of the patches defined
> in `ActionController::TestCase` to the development and production
> environment, including behaviours like disabling the threading
> implementation of `ActionController::Live`

.github/workflows/ci.yml

@@ -75,3 +75,4 @@ jobs:
           bundle exec rake
         env:
           RAILS_VERSION: ${{ matrix.rails-version }}
+          RAILS_ENV: test

lib/active_model_serializers.rb

@@ -7,12 +7,22 @@
 begin
   require 'action_controller'
   require 'action_controller/serialization'
-  require 'action_controller/serialization_test_case'
 
   ActiveSupport.on_load(:action_controller) do
     if ::ActionController::Serialization.enabled
       ActionController::Base.send(:include, ::ActionController::Serialization)
-      ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
+
+      # action_controller_test_case load hook was added in Rails 5.1
+      # https://github.com/rails/rails/commit/0510208dd1ff23baa619884c0abcae4d141fae53
+      if ActiveSupport::VERSION::STRING < '5.1'
+        require 'action_controller/serialization_test_case'
+        ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
+      else
+        ActiveSupport.on_load(:action_controller_test_case) do
+          require 'action_controller/serialization_test_case'
+          ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
+        end
+      end
     end
   end
 rescue LoadError