Added Bundle class implementation along with tests

spec/Bundle.ts

@@ -0,0 +1,58 @@
+import { Stream } from "stream";
+import { default as Bundle, filesSymbol } from "../lib/Bundle";
+
+describe("Bundle", () => {
+	let bundle: InstanceType<typeof Bundle>;
+
+	beforeEach(() => {
+		bundle = new Bundle("application/vnd.apple.pkpass");
+	});
+
+	it("should throw an error if no mime-type is specified in the constructor", () => {
+		// @ts-expect-error
+		expect(() => new Bundle()).toThrowError(
+			Error,
+			"Cannot build Bundle. MimeType is missing",
+		);
+	});
+
+	it("should expose the mime-type as public property", () => {
+		expect(bundle.mimeType).toBe("application/vnd.apple.pkpass");
+	});
+
+	it("should allow to add buffers", () => {
+		const buffer = Buffer.alloc(0);
+		bundle.addBuffer("pass.json", buffer);
+
+		expect(bundle[filesSymbol]).toEqual({ "pass.json": buffer });
+	});
+
+	it("should throw error if freezed", async () => {
+		addEmptyFilesToBundle(bundle);
+
+		await bundle.getAsBuffer();
+
+		expect(() =>
+			bundle.addBuffer("icon.png", Buffer.alloc(0)),
+		).toThrowError(Error, "Cannot add file. Bundle is closed.");
+	});
+
+	it("should return a stream with 'getAsStream'", () => {
+		addEmptyFilesToBundle(bundle);
+
+		expect(bundle.getAsStream()).toBeInstanceOf(Stream);
+	});
+
+	it("should return a buffer with 'getAsBuffer'", async () => {
+		addEmptyFilesToBundle(bundle);
+
+		expect(await bundle.getAsBuffer()).toBeInstanceOf(Buffer);
+	});
+});
+
+function addEmptyFilesToBundle(bundle: Bundle) {
+	const buffer = Buffer.alloc(0);
+	bundle.addBuffer("pass.json", buffer);
+	bundle.addBuffer("icon@2x.png", buffer);
+	bundle.addBuffer("icon@3x.png", buffer);
+}