Crashes on non-ASCII header values #430

Hello,
I've found a crash in impit when parsing HTTP response headers that contain non-ASCII characters in their values.

Steps to reproduce

Fetch the following URL using impit-node: https://www.gamesunit.de/feed/the-wait-is-over-broken-arrow-s-first-dlc-is-here-and-it-s-free-227661.html

This will show the following error

Failed to parse header value for 'last-modified': ToStrError { _priv: () }

Root cause

The server returns a last-modified header with a German-locale date string:

last-modified: Dienstag, 31. März 2026

I'm aware that strictly only ASCII characters should be used in headers part, but this situation exists and I hope the library could be more resilient on these cases.

Thanks