How do I redirect without www using Rails 3 / Rack?

Question

In Ruby on Rails 4, removing www. from any URL whilst maintaining the pathname can be achieved simply by using:

# config/routes.rb

constraints subdomain: 'www' do
  get ':any', to: redirect(subdomain: nil, path: '/%{any}'), any: /.*/
end

In contrast, adding www. to the beginning of any URL that doesn’t already have it can be achieved by:

# config/routes.rb

constraints subdomain: false do
  get ':any', to: redirect(subdomain: 'www', path: '/%{any}'), any: /.*/
end