Use UTF-8 decode without BOM for multipart/form-data

[CYBAI]

whatwg/fetch#915
web-platform-tests/wpt#17589

@jdm I think following code is the related function 👀?

servo/components/script/body.rs

Lines 173 to 200 in f626355

	fn run_form_data_algorithm(
	root: &GlobalScope,
	bytes: Vec<u8>,
	mime: &[u8],
	) -> Fallible<FetchedData> {
	let mime_str = if let Ok(s) = str::from_utf8(mime) {
	s
	} else {
	""
	};
	let mime: Mime = mime_str
	.parse()
	.map_err(|_| Error::Type("Inappropriate MIME-type for Body".to_string()))?;
	// TODO
	// ... Parser for Mime(TopLevel::Multipart, SubLevel::FormData, _)
	// ... is not fully determined yet.
	if mime.type_() == mime::APPLICATION && mime.subtype() == mime::WWW_FORM_URLENCODED {
	let entries = form_urlencoded::parse(&bytes);
	let formdata = FormData::new(None, root);
	for (k, e) in entries {
	formdata.Append(USVString(k.into_owned()), USVString(e.into_owned()));
	}
	return Ok(FetchedData::FormData(formdata));
	}
	Err(Error::Type("Inappropriate MIME-type for Body".to_string()))
	}

[jdm]

You are correct, and it's also related to https://github.com/servo/rust-url/blob/7d2c9d6ceb3307a3fad4c4bcb7f5b07b5e24c755/src/form_urlencoded.rs#L21-L58. It's not entirely clear to me what needs to be done.