Add a check to not allow `addTrailers` on an already ended http reply

[ChALkeR]

setHeader / setHeaders have a protection against a mistake:

node/lib/_http_outgoing.js

Lines 645 to 648 in adeb14a

	OutgoingMessage.prototype.setHeader = function setHeader(name, value) {
	if (this._header) {
	throw new ERR_HTTP_HEADERS_SENT('set');
	}

node/lib/_http_outgoing.js

Lines 660 to 663 in adeb14a

	OutgoingMessage.prototype.setHeaders = function setHeaders(headers) {
	if (this._header) {
	throw new ERR_HTTP_HEADERS_SENT('set');
	}

addTrailers doesn't, and just assigns to ._trailers _even if the response is already ended/sent.

node/lib/_http_outgoing.js

Lines 984 to 986 in adeb14a

	OutgoingMessage.prototype.addTrailers = function addTrailers(headers) {
	this._trailer = '';
	const keys = ObjectKeys(headers);

Note: adding this check would be a semver-major change.

E.g. fastify incorrectly attempts to do this under some conditions due to a logic mistake.