Use defined GraphQL::Types::ISO8601DateTime instead of defining similar one

Author: meganemura

test/test_query_result.rb

@@ -6,17 +6,7 @@
 require "ostruct"
 require_relative "foo_helper"
 
-class TestQueryResult < Minitest::Test
-  class DateTimeType < GraphQL::Schema::Scalar
-    def self.coerce_input(value, ctx)
-      Time.iso8601(value)
-    end
-
-    def self.coerce_result(value, ctx)
-      value.utc.iso8601
-    end
-  end
-
+class TestQueryResult < MiniTest::Test
   module NodeType
     include GraphQL::Schema::Interface
     field :id, ID, null: false
@@ -58,7 +48,7 @@ class UserType < GraphQL::Schema::Object
 
   module HumanLike
     include GraphQL::Schema::Interface
-    field :updated_at, DateTimeType, null: false
+    field :updated_at, GraphQL::Types::ISO8601DateTime, null: false
   end
 
   class PersonType < GraphQL::Schema::Object
@@ -69,7 +59,7 @@ class PersonType < GraphQL::Schema::Object
     field :last_name, String, null: true
     field :company, String, null: true
     field :homepageURL, String, null: true, method: :homepage_url, camelize: false
-    field :created_at, DateTimeType, null: false
+    field :created_at, GraphQL::Types::ISO8601DateTime, null: false
     field :hobbies, [String], null: true
     field :plan, PlanType, null: false
   end
@@ -596,7 +586,7 @@ def test_interface_within_union_values
     assert_equal Time.at(1).utc, actor.updated_at
   end
 
-  def test_date_scalar_casting
+  def test_datetime_scalar_casting
     Temp.const_set :Query, @client.parse(<<-'GRAPHQL')
       {
         me {