fix(http): decode header values as ISO-8859-1 (#3586)

* fix(http): decode response header values as ISO-8859-1

Response header values were decoded with String::from_utf8 before being
sent over IPC. Raw UTF-8 bytes produced JS strings with code points
above 255, which new Headers() rejects as invalid ByteStrings
("Type error" on WebKit), and bytes that were not valid UTF-8 failed
the fetch outright. Decode as ISO-8859-1 instead so every byte maps to a
char 0-255, matching what browsers expose for the same response.

Fixes #2898

* fix(http): add tests for ISO-8859-1 response header decoding

Cover the #2898 Content-Disposition case and non-UTF-8 header bytes on
the same conversion fetch_send uses, so a revert to UTF-8 fails the suite.

* request headers, e2e test

---------

Co-authored-by: Lucas Nogueira <lucas@tauri.app>

.changes/http-non-utf8-headers.md

@@ -0,0 +1,6 @@
+---
+"http": patch
+"http-js": patch
+---
+
+Fix `fetch` throwing a `TypeError` when a response header value contains non-ASCII bytes (e.g. a UTF-8 encoded `Content-Disposition` filename). Header values are now treated as byte strings like in a browser: response header values are decoded as ISO-8859-1 so the raw bytes are preserved as a valid `ByteString` for `new Headers()`, and request header values are sent as their raw bytes (one byte per character) instead of being UTF-8 encoded. A request header value with a character such as `é` is now sent as the single byte `0xE9`, as browsers do.