fix: do not modify the input packet upon encoding

Note: this issue has existed since Socket.IO v1.0 (see [1]), because
the `deconstructPacket()` method also mutates its input argument.

This also explains why some adapters (like [2]) need to use
`process.nextTick()` when extending the `broadcast()` method, because
`Adapter.broadcast()` calls `Encoder.encode()` ([3]).

Related:

- https://github.com/socketio/socket.io/issues/4374
- https://github.com/socketio/socket.io-mongo-adapter/issues/10

[1]: https://github.com/socketio/socket.io-parser/commit/299849b00294c3bc95817572441f3aca8ffb1f65
[2]: https://github.com/socketio/socket.io-postgres-adapter/blob/0.3.0/lib/index.ts#L587-L590
[3]: https://github.com/socketio/socket.io-adapter/blob/2.4.0/lib/index.ts#L148