Error message for invalid headers could be improved

[benmccann]

Bug Description

I'm getting the error message:

TypeError: Headers contructor: init is a symbol, which cannot be converted to a DOMString.

Reproducible By

const headers = new Headers({'x-fake-header': Symbol('why is this here?')})

Expected Behavior

contructor should not be typo'd

"init is a symbol" is incorrect. It contains a symbol, but isn't one

Logs & Screenshots

TypeError: Headers contructor: init is a symbol, which cannot be converted to a DOMString.
    at webidl.errors.exception (node:internal/deps/undici/undici:3384:14)
    at webidl.converters.DOMString (node:internal/deps/undici/undici:3650:29)
    at webidl.converters.ByteString (node:internal/deps/undici/undici:3658:35)
    at Object.record<ByteString, ByteString> (node:internal/deps/undici/undici:3568:32)
    at webidl.converters.HeadersInit (node:internal/deps/undici/undici:8695:67)

Environment

Tested on Node.js v20.17.0 and v22.11.0

Additional context

Users were running into this in SvelteKit projects. I'm not quite sure how/why yet, but I think maybe connect (which is used by vite) has something like this in its headers and the headers are being copied from there