fix: fix payload encoding for v3 clients

The v3 parser (used for compatibility with older clients) was broken
during the migration to TypeScript ([1]).

This was not caught in the test suite because the Node.js client does
not support binary packet in polling mode (packets are base64-encoded).

[1]: c0d6eaa

Backported from 6.0.x branch: 3f42262

lib/parser-v3/utf8.ts

@@ -203,7 +203,7 @@ function utf8decode(byteString, opts) {
 	return ucs2encode(codePoints);
 }
 
-export default {
+module.exports = {
 	version: '2.1.2',
 	encode: utf8encode,
 	decode: utf8decode

test/parser.js

@@ -0,0 +1,22 @@
+const expect = require("expect.js");
+const parser = require("../build/parser-v3/index.js");
+
+describe("parser", () => {
+  it("properly encodes a mixed payload", done => {
+    parser.encodePayload(
+      [
+        { type: "message", data: "€€€€" },
+        { type: "message", data: Buffer.from([1, 2, 3]) }
+      ],
+      true,
+      encoded => {
+        expect(encoded).to.be.a(Buffer);
+
+        parser.decodePayload(encoded, decoded => {
+          expect(decoded.data).to.eql("€€€€");
+          done();
+        });
+      }
+    );
+  });
+});