fix: preserve case in setCharset

[tushar73-jet]

This change fixes an issue where utils.setCharset would incorrectly convert the content type to lowercase when adding a charset.

This PR preserves the original case of the content type, as discussed in the issue. It also includes a new test case to verify this behaviour.

Fixes #6613

[bjohansebas]

Thank you for your contribution, but I’m going to close this, as the current behavior will be maintained.

see #6613 (comment) and #6613 (comment)

[tushar73-jet]

Hi @bjohansebas, thank you for reviewing my PR.
Since this fix won’t be merged, could you please assign me another issue or task where I can contribute?
I’m eager to help and would appreciate any guidance on areas where contributions are needed.
Thank you!

[bjohansebas]

Feel free to pick another issue or review other PRs. We don’t assign PRs

this is open source, and we contribute when we have time.

[tushar73-jet]

Got it, thanks for clarifying! I’ll explore the open issues to contribute further. Appreciate your time.