Undici Dispatcher's request.headersList.entries changed into an object

[hansottowirtz]

This comment kind of explains itself:

miniflare/packages/core/src/standards/http.ts

Lines 797 to 800 in 78c2e0a

	// Note: I'm fully expecting this to break in future undici versions
	// and need to be updated, but that's why we pin our undici version and
	// have tests
	assert(Array.isArray(headers));

This breaks in the latest Undici version.

It's now a Record<string, string>: https://github.com/nodejs/undici/blob/main/lib/fetch/headers.js#L176-L186