use ascii instead of utf-8

[Uzlopak]

writeEarlyHints on node uses ascii instead of utf-8. in node the strings get actually also validated by a regex. That regex is imho validating implicitly that it is ascii. Maybe we need to url_encode href to be sure we dont send garbage?

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

[Uzlopak]

closing in favor of #5