Revert dropping MultiJson to fix Time serialization regression (#405)

JSON.dump bypasses ActiveSupport's Hash#to_json override, causing
Time values to serialize as "2026-04-16 10:55:10 UTC" instead of
ISO 8601 format. Restore MultiJson.dump to match 1.0.1 behavior.

Fixes #403

Author: numbata

CHANGELOG.md

@@ -7,6 +7,7 @@
 #### Fixes
 
 * Your contribution here.
+* [#405](https://github.com/ruby-grape/grape-entity/pull/405): Fix `Time` serialization regression by reverting #385 and restoring `MultiJson.dump` - [@numbata](https://github.com/numbata).
 * [#402](https://github.com/ruby-grape/grape-entity/pull/402): Remove `Json::ParseError` assignment - [@olivier-thatch](https://github.com/olivier-thatch).
 
 ### 1.0.3 (2026-04-15)

grape-entity.gemspec

@@ -25,6 +25,7 @@ Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.0'
 
   s.add_dependency 'activesupport', '>= 3.0.0'
+  s.add_dependency 'multi_json', '>= 1.0'
 
   s.files         = Dir['lib/**/*.rb', 'CHANGELOG.md', 'LICENSE', 'README.md']
   s.require_paths = ['lib']

lib/grape_entity/entity.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'grape_entity/json'
+require 'multi_json'
 
 module Grape
   # An Entity is a lightweight structure that allows you to easily
@@ -594,7 +594,7 @@ def is_defined_in_entity?(attribute)
 
     def to_json(options = {})
       options = options.to_h if options&.respond_to?(:to_h)
-      Grape::Entity::Json.dump(serializable_hash(options))
+      MultiJson.dump(serializable_hash(options))
     end
 
     def to_xml(options = {})

lib/grape_entity/json.rb

@@ -1880,6 +1880,23 @@ class NoPathCharacterEntity < Grape::Entity
       it 'returns a json' do
         expect(fresh_class.new(model).to_json).to eq(JSON.generate(attributes.slice(:name)))
       end
+
+      it 'serializes Time values via as_json' do
+        fresh_class.expose :birthday
+        entity = fresh_class.new(model)
+        json = entity.to_json
+        parsed = JSON.parse(json)
+        expect(parsed['birthday']).to eq(attributes[:birthday].as_json)
+      end
+
+      it 'serializes Time values in nested exposures via as_json' do
+        fresh_class.expose :details do
+          fresh_class.expose :birthday
+        end
+        entity = fresh_class.new(model)
+        parsed = JSON.parse(entity.to_json)
+        expect(parsed['details']['birthday']).to eq(attributes[:birthday].as_json)
+      end
     end
 
     describe '#inspect' do