Fix for frozen string literal to prepare for ruby 3.4

[chaadow]

Cf. https://gist.github.com/fxn/bf4eed2505c76f4fca03ab48c43adc72

Hi,

This is a simple fix for a case of a string being mutated while frozen ( with frozen_string_literal true)

I believe to enable this gem to be compliant with the future in ruby, and following the recommandations of the aforementionned gist, It would be good to enable the frozen string literal RUBYOPTion in CI