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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[chaadow]

@manisha1997 thank you, could you please merge and cut a release please 🙏🏼

[chaadow]

@manisha1997 can you please release on RubyGems please??

[chaadow]

here: https://rubygems.org/gems/ruby_http_client

[manisha1997]

Hello!
We are trying to do the release, we faced some issue during the release. Will try to release as soon as possible.

[manisha1997]

Released version 3.6.0

[a5-stable]

@manisha1997
Thank you for the release! I’m glad to see it.
It looks like the release notes might refer to the wrong PR:
https://github.com/sendgrid/ruby-http-client/releases/tag/3.6.0

[manisha1997]

Hey!
Apologies for the release notes.
They were added manually.