Support frozen string literals

[pat]

This is to avoid deprecation warnings with the forthcoming change in Ruby 3.4 where modifying a string literal will cause a deprecation warning by default.
ruby/ruby@12be40a

Granted, it only impacts situations where empty strings are passed through, but I figure better to cover that off than expect consumers of this gem to have their own logic.

Also, doesn't hurt to always be testing against this to avoid any regressions.