refactor: use lowercase for charset

[Fdawgs]

Whilst the content-type response header is case-insensitive, lowercase is preferred, as per RFC 9110 Section 8.3.1.

It's very rare to see any sites using uppercase anymore.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[Fdawgs]

I've noticed in that RFC it also states that no whitespace is also preferred, so text/plain;charset=utf-8 over text/plain; charset=utf-8. Is that also worth changing here?