Include missing module in tag_helper

Since 6857415187 we are using #safe_join to
join the content when an Array is given, so we must include the dependent
module here to make sure it's available when this module is used alone.

This was making Simple Form tests to fail with current master due to the
missing dependency.

actionview/lib/action_view/helpers/tag_helper.rb

@@ -9,6 +9,7 @@ module ActionView
     module TagHelper
       extend ActiveSupport::Concern
       include CaptureHelper
+      include OutputSafetyHelper
 
       BOOLEAN_ATTRIBUTES = %w(disabled readonly multiple checked autobuffer
                            autoplay controls loop selected hidden scoped async