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/adapter/json_api/linked_test.rb

@@ -0,0 +1,40 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class Adapter
+      class JsonApi
+        class LinkedTest < Minitest::Test
+          def setup
+            @author = Author.new(id: 1, name: 'Steve K.')
+            @first_post = Post.new(id: 1, title: 'Hello!!', body: 'Hello, world!!')
+            @second_post = Post.new(id: 2, title: 'New Post', body: 'Body')
+            @first_post.comments = []
+            @second_post.comments = []
+            @first_post.author = @author
+            @second_post.author = @author
+            @author.posts = [@first_post, @second_post]
+
+            @serializer = ArraySerializer.new([@first_post, @second_post])
+            @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(@serializer, include: 'author,comments')
+          end
+
+          def test_include_multiple_posts_and_linked
+            @first_comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
+            @second_comment = Comment.new(id: 2, body: 'ZOMG ANOTHER COMMENT')
+            @first_post.comments = [@first_comment, @second_comment]
+            @first_comment.post = @first_post
+            @first_comment.author = nil
+            @second_comment.post = @first_post
+            @second_comment.author = nil
+            assert_equal([
+                           { title: "Hello!!", body: "Hello, world!!", id: "1", links: { comments: ['1', '2'], author: "1" } },
+                           { title: "New Post", body: "Body", id: "2", links: { comments: [], :author => "1" } }
+                         ], @adapter.serializable_hash[:posts])
+            assert_equal({ :comments => [{ :id => "1", :body => "ZOMG A COMMENT" }, { :id => "2", :body => "ZOMG ANOTHER COMMENT" }], :authors => [{ :id => "1", :name => "Steve K." }] }, @adapter.serializable_hash[:linked])
+          end
+        end
+      end
+    end
+  end
+end