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]: https://github.com/socketio/engine.io/commit/c0d6eaa1ba1291946dc8425d5f533d5f721862dd

Backported from 6.0.x branch: https://github.com/socketio/engine.io/commit/3f42262fd27a77a7383cdbb44ede7c6211a9782b