Include 'linked' member for json-api collections

The options passed to the render are partitioned into adapter options
and serializer options. 'include' and 'root' are sent to the adapter,
not sure what options would go directly to serializer, but leaving this
in until I understand that better.

test/action_controller/json_api_linked_test.rb

@@ -0,0 +1,104 @@
+require 'test_helper'
+
+module ActionController
+  module Serialization
+    class JsonApiLinkedTest < ActionController::TestCase
+      class MyController < ActionController::Base
+        def setup_post
+          @author = Author.new(id: 1, name: 'Steve K.')
+          @author.posts = []
+          @author2 = Author.new(id: 2, name: 'Anonymous')
+          @author2.posts = []
+          @post = Post.new(id: 1, title: 'New Post', body: 'Body')
+          @first_comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
+          @second_comment = Comment.new(id: 2, body: 'ZOMG ANOTHER COMMENT')
+          @post.comments = [@first_comment, @second_comment]
+          @post.author = @author
+          @first_comment.post = @post
+          @first_comment.author = @author2
+          @second_comment.post = @post
+          @second_comment.author = nil
+        end
+
+        def with_json_api_adapter
+          old_adapter = ActiveModel::Serializer.config.adapter
+          ActiveModel::Serializer.config.adapter = :json_api
+          yield
+        ensure
+          ActiveModel::Serializer.config.adapter = old_adapter
+        end
+
+        def render_resource_without_include
+          with_json_api_adapter do
+            setup_post
+            render json: @post
+          end
+        end
+
+        def render_resource_with_include
+          with_json_api_adapter do
+            setup_post
+            render json: @post, include: 'author'
+          end
+        end
+
+        def render_resource_with_nested_include
+          with_json_api_adapter do
+            setup_post
+            render json: @post, include: 'comments.author'
+          end
+        end
+
+        def render_collection_without_include
+          with_json_api_adapter do
+            setup_post
+            render json: [@post]
+          end
+        end
+
+        def render_collection_with_include
+          with_json_api_adapter do
+            setup_post
+            render json: [@post], include: 'author,comments'
+          end
+        end
+      end
+
+      tests MyController
+
+      def test_render_resource_without_include
+        get :render_resource_without_include
+        response = JSON.parse(@response.body)
+        refute response.key? 'linked'
+      end
+
+      def test_render_resource_with_include
+        get :render_resource_with_include
+        response = JSON.parse(@response.body)
+        assert response.key? 'linked'
+        assert_equal 1, response['linked']['authors'].size
+        assert_equal 'Steve K.', response['linked']['authors'].first['name']
+      end
+
+      def test_render_resource_with_nested_include
+        get :render_resource_with_nested_include
+        response = JSON.parse(@response.body)
+        assert response.key? 'linked'
+        assert_equal 1, response['linked']['authors'].size
+        assert_equal 'Anonymous', response['linked']['authors'].first['name']
+      end
+
+      def test_render_collection_without_include
+        get :render_collection_without_include
+        response = JSON.parse(@response.body)
+        refute response.key? 'linked'
+      end
+
+      def test_render_collection_with_include
+        get :render_collection_with_include
+        response = JSON.parse(@response.body)
+        assert response.key? 'linked'
+      end
+    end
+  end
+end