use railtie to set default sanitizer

lib/rails-html-sanitizer.rb

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

lib/railtie.rb

@@ -0,0 +1,15 @@
+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