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

test/adapter/json_api/has_many_test.rb

@@ -35,10 +35,22 @@ module ActiveModel
 
           def test_includes_linked_comments
             @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(@serializer, include: 'comments')
-            assert_equal([
-                           {id: "1", body: 'ZOMG A COMMENT'},
-                           {id: "2", body: 'ZOMG ANOTHER COMMENT'}
-                         ], @adapter.serializable_hash[:linked][:comments])
+            expected = [{
+              id: "1",
+              body: 'ZOMG A COMMENT',
+              links: {
+                post: "1",
+                author: nil
+              }
+            }, {
+              id: "2",
+              body: 'ZOMG ANOTHER COMMENT',
+              links: {
+                post: "1",
+                author: nil
+              }
+            }]
+            assert_equal expected, @adapter.serializable_hash[:linked][:comments]
           end
 
           def test_no_include_linked_if_comments_is_empty

test/adapter/json_api/linked_test.rb

@@ -35,10 +35,42 @@ module ActiveModel
                            { 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." }],
-                           :bios=>[{:id=>"1", :content=>"AMS Contributor"}] }, @adapter.serializable_hash[:linked])
+
+
+            expected = {
+              comments: [{
+                id: "1",
+                body: "ZOMG A COMMENT",
+                links: {
+                  post: "1",
+                  author: nil
+                }
+              }, {
+                id: "2",
+                body: "ZOMG ANOTHER COMMENT",
+                links: {
+                  post: "1",
+                  author: nil
+                }
+              }],
+              authors: [{
+                id: "1",
+                name: "Steve K.",
+                links: {
+                  posts: ["1", "2"],
+                  roles: [],
+                  bio: "1"
+                }
+              }],
+              bios: [{
+                id: "1",
+                content: "AMS Contributor",
+                links: {
+                  author: "1"
+                }
+              }]
+            }
+            assert_equal expected, @adapter.serializable_hash[:linked]
           end
         end
       end