Add support for toplevel JSON API links.

test/adapter/json_api/links_test.rb

@@ -0,0 +1,32 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    module Adapter
+      class JsonApi
+        class LinksTest < Minitest::Test
+          def setup
+            @post = Post.new(id: 1337, comments: [], author: nil)
+          end
+
+          def test_toplevel_links
+            hash = ActiveModel::SerializableResource.new(
+              @post,
+              adapter: :json_api,
+              links: {
+                self: {
+                  href: '//posts'
+                }
+              }).serializable_hash
+            expected = {
+              self: {
+                href: '//posts'
+              }
+            }
+            assert_equal(expected, hash[:links])
+          end
+        end
+      end
+    end
+  end
+end