fix: content-disposition header parsing

[climba03003]

Fixes #1903
I am not sure if it is good to mutate in onHeaderValue.

But I found it should be the most optimal place to do it.
Trans-code the header value in other place would result in heavy boating code.
For example, it would duplicate the logic if modify in parseHeaders.

undici/lib/core/util.js

Lines 212 to 235 in 7827031

	function parseHeaders (headers, obj = {}) {
	for (let i = 0; i < headers.length; i += 2) {
	const key = headers[i].toString().toLowerCase()
	let val = obj[key]
	if (!val) {
	if (Array.isArray(headers[i + 1])) {
	obj[key] = headers[i + 1]
	} else {
	obj[key] = headers[i + 1].toString()
	}
	} else {
	if (!Array.isArray(val)) {
	val = [val]
	obj[key] = val
	}
	val.push(headers[i + 1].toString())
	}
	}
	return obj
	}
	function parseRawHeaders (headers) {
	return headers.map(header => header.toString())
	}

[ronag]

You can't process headers in onHeaderValue as they might be incomplete. It needs to be done in onHeadersComplete

[ronag]

I helped out a little bit. Hope you don't mind.

[climba03003]

Thanks for the helps.
I didn't do it in lib/core/util.js in first place because the code is neat and tidy.