Ruby 2.4 and later support native Unicode case mappings

Here is only place where we use `mb_chars` internally.

actionview/lib/action_view/helpers/tags/base.rb

@@ -138,7 +138,7 @@ def sanitized_method_name
           end
 
           def sanitized_value(value)
-            value.to_s.gsub(/[\s\.]/, "_").gsub(/[^-[[:word:]]]/, "").mb_chars.downcase.to_s
+            value.to_s.gsub(/[\s\.]/, "_").gsub(/[^-[[:word:]]]/, "").downcase
           end
 
           def select_content_tag(option_tags, options, html_options)