fix: frozen string literal issue for ruby 3.4.0

[chaadow]

Fixes

Fixes https://github.com/sendgrid/ruby-http-client/issues/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 https://github.com/fog/fog-aws/pull/709

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [ ] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [ ] I have read the Contribution Guidelines and my PR follows them
• [ ] I have titled the PR appropriately
• [ ] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

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