Fix ActiveSupport::Inflector.pluralize behavior for words that consist of non-ASCII characters

This PR fixes behavior of `ActiveSupport::Inflector.pluralize`.

The code below returns "猫" in 3.2, but "猫s" in 4.0.
```ruby
ActiveSupport::Inflector.inflections do |inflect|
  inflect.uncountable "猫"
end
"猫".pluralize
```