Includes links inside of linked resources

According to
http://jsonapi.org/format/#document-structure-resource-objects,

> Resource objects have the same internal structure, regardless of
> whether they represent primary or linked resources.

And then
http://jsonapi.org/format/#document-structure-resource-object-attributes,

> There are four reserved keys in resource objects:
>
> "id"
> "type"
> "href"
> "links"

This commits includes `links` inside of linked resources.

test/adapter/json_api/belongs_to_test.rb

@@ -32,7 +32,16 @@ module ActiveModel
 
           def test_includes_linked_post
             @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(@serializer, include: 'post')
-            assert_equal([{id: "42", title: 'New Post', body: 'Body'}], @adapter.serializable_hash[:linked][:posts])
+            expected = [{
+              id: "42",
+              title: 'New Post',
+              body: 'Body',
+              links: {
+                comments: ["1"],
+                author: "1"
+              }
+            }]
+            assert_equal expected, @adapter.serializable_hash[:linked][:posts]
           end
 
           def test_include_nil_author