Drop multijson dependency (#385)

* Replace Multijson by Grape::Entity::Json
Add require spec_helper in rspec file
Add spec

* Fix rubocop

.rspec

@@ -1,3 +1,4 @@
+--require spec_helper
 --color
 --profile
 --format documentation

grape-entity.gemspec

@@ -17,8 +17,6 @@ Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.0'
 
   s.add_runtime_dependency 'activesupport', '>= 3.0.0'
-  # FIXME: remove dependecy
-  s.add_runtime_dependency 'multi_json', '>= 1.3.2'
 
   s.files         = `git ls-files`.split("\n")
   s.test_files    = `git ls-files -- {test,spec}/*`.split("\n")

lib/grape_entity/entity.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'multi_json'
+require 'grape_entity/json'
 
 module Grape
   # An Entity is a lightweight structure that allows you to easily
@@ -558,7 +558,7 @@ def is_defined_in_entity?(attribute)
 
     def to_json(options = {})
       options = options.to_h if options&.respond_to?(:to_h)
-      MultiJson.dump(serializable_hash(options))
+      Grape::Entity::Json.dump(serializable_hash(options))
     end
 
     def to_xml(options = {})

lib/grape_entity/json.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+module Grape
+  class Entity
+    if defined?(::MultiJson)
+      Json = ::MultiJson
+    else
+      Json = ::JSON
+      Json::ParseError = Json::ParserError
+    end
+  end
+end

spec/grape_entity/entity_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'spec_helper'
 require 'ostruct'
 
 describe Grape::Entity do
@@ -1755,6 +1754,16 @@ class NoPathCharacterEntity < Grape::Entity
       end
     end
 
+    describe '#to_json' do
+      before do
+        fresh_class.expose :name
+      end
+
+      it 'returns a json' do
+        expect(fresh_class.new(model).to_json).to eq(JSON.generate(attributes.slice(:name)))
+      end
+    end
+
     describe '#inspect' do
       before do
         fresh_class.class_eval do

spec/grape_entity/exposure/nesting_exposure/nested_exposures_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'spec_helper'
-
 describe Grape::Entity::Exposure::NestingExposure::NestedExposures do
   subject(:nested_exposures) { described_class.new([]) }
 

spec/grape_entity/exposure/represent_exposure_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'spec_helper'
-
 describe Grape::Entity::Exposure::RepresentExposure do
   subject(:exposure) { described_class.new(:foo, {}, {}, double, double) }
 

spec/grape_entity/exposure_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'spec_helper'
-
 describe Grape::Entity::Exposure do
   let(:fresh_class) { Class.new(Grape::Entity) }
   let(:model) { double(attributes) }

spec/grape_entity/hash_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'spec_helper'
-
 describe Grape::Entity do
   it 'except option for nested entity', :aggregate_failures do
     module EntitySpec

spec/grape_entity/json_spec.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+describe Grape::Entity::Json do
+  subject { described_class }
+
+  it { is_expected.to eq(JSON) }
+end

spec/grape_entity/options_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'spec_helper'
-
 describe Grape::Entity::Options do
   module EntitySpec
     class Crystalline