Bring back assert_serializer for controller testing

The `assert_serializer` test helper was added in 0.9.0.apha1[1],
and was not included in 0.10.

This patch brings back the `assert_serializer` test helper. This is the last
revision[2] that has the helper. The original helper was used as base.

[1]: https://github.com/rails-api/active_model_serializers/pull/596
[2]: 610aeb2e92

- Create the AssertSerializer

- Use the Test namespace

- Make the tests pass on the Rails master
  - Rails 5 does not include `assert_template` but we need this on the tests of
the helper.
  - This add the `rails-controller-testing` to keep support on `assert_template`.

- Only load test helpers in the test environment

test/active_model_serializers/test/serializer_test.rb

@@ -0,0 +1,73 @@
+require 'test_helper'
+require 'rails-controller-testing' if Rails::VERSION::MAJOR >= 5
+
+module ActiveModelSerializers
+  module Test
+    class SerializerTest < ActionController::TestCase
+      include ActiveModelSerializers::Test::Serializer
+
+      class MyController < ActionController::Base
+        def render_using_serializer
+          render json: Profile.new(name: 'Name 1', description: 'Description 1', comments: 'Comments 1')
+        end
+
+        def render_text
+          render text: 'ok'
+        end
+
+        def render_template
+          prepend_view_path './test/fixtures'
+          render template: 'template'
+        end
+      end
+
+      tests MyController
+
+      def test_supports_specifying_serializers_with_a_serializer_class
+        get :render_using_serializer
+        assert_serializer ProfileSerializer
+      end
+
+      def test_supports_specifying_serializers_with_a_regexp
+        get :render_using_serializer
+        assert_serializer(/\AProfile.+\Z/)
+      end
+
+      def test_supports_specifying_serializers_with_a_string
+        get :render_using_serializer
+        assert_serializer 'ProfileSerializer'
+      end
+
+      def test_supports_specifying_serializers_with_a_symbol
+        get :render_using_serializer
+        assert_serializer :profile_serializer
+      end
+
+      def test_supports_specifying_serializers_with_a_nil
+        get :render_text
+        assert_serializer nil
+      end
+
+      def test_raises_descriptive_error_message_when_serializer_was_not_rendered
+        get :render_using_serializer
+        e = assert_raise ActiveSupport::TestCase::Assertion do
+          assert_serializer 'PostSerializer'
+        end
+        assert_match 'expecting <"PostSerializer"> but rendering with <["ProfileSerializer"]>', e.message
+      end
+
+      def test_raises_argument_error_when_asserting_with_invalid_object
+        get :render_using_serializer
+        e = assert_raise ArgumentError do
+          assert_serializer Hash
+        end
+        assert_match 'assert_serializer only accepts a String, Symbol, Regexp, ActiveModel::Serializer, or nil', e.message
+      end
+
+      def test_does_not_overwrite_notification_subscriptions
+        get :render_template
+        assert_template 'template'
+      end
+    end
+  end
+end

test/fixtures/template.html.erb

@@ -0,0 +1 @@
+<p>Hello.</p>