use railtie to set default sanitizer

* add rails as a test dependency to fix tests

Gemfile

@@ -18,3 +18,7 @@ end
 # specify gem versions for old rubies
 gem "nokogiri", RUBY_VERSION < "2.1" ? "~> 1.6.0" : ">= 1.7"
 gem "activesupport", RUBY_VERSION < "2.2.2" ? "~> 4.2.0" : ">= 5"
+
+group :test do
+  gem "rails", "~> 7.1.1"
+end

lib/rails-html-sanitizer.rb

@@ -59,3 +59,5 @@ def deprecate_option(name)
     end
   end
 end
+
+require_relative "railtie"

lib/railtie.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require "rails"
+require "rails/html/sanitizer"
+
+module Rails
+  module HTML
+    class Sanitizer
+      class Railtie < Rails::Railtie # :nodoc:
+        initializer "rails_html_sanitizer.sanitizer_vendor" do |app|
+          ActiveSupport.on_load(:action_view) do
+            ActionView::Helpers::SanitizeHelper.sanitizer_vendor = Rails::HTML4::Sanitizer
+          end
+        end
+      end
+    end
+  end
+end