fix: frozen string literal issue for ruby 3.4.0

[chaadow]

Fixes

Fixes #136

To prepare for ruby 3.4.0 and making strings frozen by default, I've stumbled upon these string mutations in my specs while trying to send an email

future of frozen string literals in ruby : https://gist.github.com/fxn/bf4eed2505c76f4fca03ab48c43adc72

similar to fog/fog-aws#709

Checklist

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