avoid frozen string

[espen]

Fixes #73

[dennispaagman]

hey @GBH 👋

In preparation to freeze all strings by default, Ruby is emitting deprecation warnings when strings are modified without explicitly marking them.

Here is som reading on the topic:

• https://www.prateekcodes.dev/ruby-34-frozen-string-literals-rails-upgrade-guide/
• https://bugs.ruby-lang.org/issues/20205

This PR (ty @espen 🙌) will fix those issues for this gem, making it future proof for future Ruby versions.

Are you willing to have a look at this? I'm not sure if you still feel like maintaining this gem as the last release was 7 years ago. The gem is still quite popular, so I think a lot people will run into this issue at some point with future Ruby versions.

If you want I can put in some time to get it up to date (testing against modern rubies and rails versions, for example). Let me know!