Explicitly require base64 to fix uninitialized constant Base64

on Ruby 3.0 on Rails 7.0
https://github.com/amatsuda/activeresource/actions/runs/3969882547/jobs/6804943801#step:4:65

Author: amatsuda

test/abstract_unit.rb

@@ -12,6 +12,7 @@
 require "active_support/test_case"
 require "setter_trap"
 require "active_support/logger"
+require "base64"
 
 ActiveSupport::TestCase.test_order = :random if ActiveSupport::TestCase.respond_to?(:test_order=)
 ActiveResource::Base.logger = ActiveSupport::Logger.new("#{File.dirname(__FILE__)}/debug.log")