Fix: resource object identifier with nil id excludes id

lib/active_model_serializers/adapter/json_api/resource_identifier.rb

@@ -38,8 +38,11 @@ module ActiveModelSerializers
         end
 
         def as_json
-          return nil if id.blank?
-          { id: id, type: type }
+          if id.blank?
+            { type: type }
+          else
+            { id: id.to_s, type: type }
+          end
         end
 
         protected

test/adapter/json_api/type_test.rb

@@ -128,6 +128,13 @@ module ActiveModelSerializers
           assert_equal actual, expected
         end
 
+        def test_blank_id
+          @model.id = nil
+          actual = actual_resource_identifier_object(AuthorSerializer)
+          expected = { type: expected_model_type }
+          assert_equal actual, expected
+        end
+
         def test_id_defined_on_serializer
           actual = actual_resource_identifier_object(WithDefinedIdSerializer)
           expected = { id: 'special_id', type: expected_model_type }