String inflections should use I18n.locale if locale is not specified

When we set languages other than English (`:en`), we expect every word to be translated to the language we set. However, `String#pluralize` and `String#singularize` still use `:en` when locale is not specified. This means we always need to specify locale every time we use one of these methods.

To avoid that mess, let `String#pluralize` and `String#singularize` use `I18n.locale` by default so everything will be translated properly without specifying locale.