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/serializers/associations_test.rb

@@ -30,6 +30,7 @@ module ActiveModel
         @comment = Comment.new({ id: 1, body: 'ZOMG A COMMENT' })
         @post.comments = [@comment]
         @comment.post = @post
+        @comment.author = nil
         @post.author = @author
         @author.posts = [@post]
 
@@ -47,11 +48,17 @@ module ActiveModel
       end
 
       def test_has_one
-        assert_equal({post: {type: :belongs_to, options: {}}}, @comment_serializer.class._associations)
+        assert_equal({post: {type: :belongs_to, options: {}}, :author=>{:type=>:belongs_to, :options=>{}}}, @comment_serializer.class._associations)
         @comment_serializer.each_association do |name, serializer, options|
-          assert_equal(:post, name)
-          assert_equal({}, options)
-          assert_kind_of(PostSerializer, serializer)
+          if name == :post
+            assert_equal({}, options)
+            assert_kind_of(PostSerializer, serializer)
+          elsif name == :author
+            assert_equal({}, options)
+            assert_nil serializer
+          else
+            flunk "Unknown association: #{name}"
+          end
         end
       end
     end