Fix Time serialization regression introduced by dropping MultiJson

JSON.dump bypasses ActiveSupport's Hash#to_json override, causing
Time values to serialize via Time#to_s ("2026-04-16 10:55:10 UTC")
instead of ActiveSupport's as_json ("2026-04-16T10:55:10.597Z").

Replace Json.dump with Hash#to_json which properly delegates through
ActiveSupport and remove the now-unused Grape::Entity::Json constant.

Fixes #403

Author: numbata

lib/grape_entity/entity.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'multi_json'
-
 module Grape
   # An Entity is a lightweight structure that allows you to easily
   # represent data from your application in a consistent and abstracted
@@ -594,7 +592,13 @@ def is_defined_in_entity?(attribute)
 
     def to_json(options = {})
       options = options.to_h if options&.respond_to?(:to_h)
+<<<<<<< HEAD
       MultiJson.dump(serializable_hash(options))
+||||||| parent of 374a021 (Fix Time serialization regression introduced by dropping MultiJson)
+      Grape::Entity::Json.dump(serializable_hash(options))
+=======
+      serializable_hash(options).to_json
+>>>>>>> 374a021 (Fix Time serialization regression introduced by dropping MultiJson)
     end
 
     def to_xml(options = {})