Revert "fix: undefined will stay undefined instead of null"

Changing the encoding of `undefined` is a breaking change. 
I am postponing this so we can bundle this with other breaking changes.
It’s still a good idea to change this eventually.

This reverts commit 83af274.

__test__/bugs.spec.ts

@@ -3,6 +3,11 @@ import { expect, describe, it } from "@jest/globals";
 import { packAndUnpack } from "./util";
 
 describe("Bugs", () => {
+	describe("Objects", () => {
+		it("replaces undefined with null ", async () => {
+			expect(await packAndUnpack(undefined)).toBe(null);
+		});
+	});
 	describe("Numbers", () => {
 		it("gives back wrong value on INT64_MAX ", async () => {
 			expect(await packAndUnpack(0x7fffffffffffffff)).toBe(

__test__/objects.spec.ts

@@ -31,9 +31,6 @@ describe("Binarypack", () => {
 	it("should keep null", async () => {
 		expect(await packAndUnpack(null)).toEqual(null);
 	});
-	it("should keep undefined", async () => {
-		expect(await packAndUnpack(undefined)).toEqual(undefined);
-	});
 
 	it("should transfer Uint8Array views correctly", async () => {
 		const arr = new Uint8Array(8);

lib/binarypack.ts

@@ -312,7 +312,7 @@ export class Packer {
 				this._bufferBuilder.append(0xc2);
 			}
 		} else if (value === undefined) {
-			this._bufferBuilder.append(0xc1);
+			this._bufferBuilder.append(0xc0);
 		} else if (typeof value === "object") {
 			if (value === null) {
 				this._bufferBuilder.append(0xc0);