Fix Rails 5.1 compatibility: Remove ActiveModel::Attributes

ActiveModel::Attributes is only available in Rails 5.2+. For Rails 5.1
compatibility, use attr_accessor instead.

Changes:
- Remove 'include ActiveModel::Attributes'
- Replace 'attribute :id, :integer' with 'attr_accessor :id'
- Add explicit initialize method

This ensures the test fixtures work with Rails 5.1.7 and above.

Author: takaokouji

test/fixtures/active_record.rb

@@ -2695,11 +2695,14 @@ class RobotResource < ::JSONAPI::Resource
 # ActiveModel class without ActiveRecord
 class SimpleModel
   include ActiveModel::Model
-  include ActiveModel::Attributes
 
-  attribute :id, :integer
-  attribute :name, :string
-  attribute :description, :string
+  attr_accessor :id, :name, :description
+
+  def initialize(attributes = {})
+    @id = attributes[:id]
+    @name = attributes[:name]
+    @description = attributes[:description]
+  end
 
   # Simple in-memory store for testing
   @@store = {}