socketioxide v4 binary bug reproducer

This reproduces an issue where socketioxide, when acting as a v4 server, fails to receive binary events properly. The Vec<u8> passed to the handler contains the correct binary data, but it is prefixed with a 0x4 byte.

To demonstrate

0. Prerequisites:
   • A cargo/rust compiler environment
   • JDK 11 or greater, and maven
1. In one terminal, run make run-server
2. In another terminal, run make run-client
3. Assuming both have compiled and run successfully, switch back to the first terminal and watch for output.

What actually happens

We see:

Got binary data: [4, 98, 105, 110, 97, 114, 121]

What's expected

But we should see:

Got binary data: [98, 105, 110, 97, 114, 121]

Notes

When socketioxide acts as a v5 server (change the version of the Java socketio client in client/pom.xml to 2.1.0), socketioxide receives binary frames as expected.

If the client connects using the HTTP polling transport, the issue does not occur. (Open Main.java and change the options.transports line so it only contains Polling.NAME to see this in action.)

Other server implementations, such as the socket.io-server-java and JS socket.io 2.5.0 see the binary frames properly.