fix: make sure to add / prefix to the namespace name

[tomruk]

The kind of change this PR does introduce

• a new feature
• a bug fix
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

Trying to connect to a namespace without the slash prefix ends up with a decoder error (error occurs on the server side):

Error: invalid payload
    at Decoder.decodeString (/home/user/dojo/socket.io/node_modules/.pnpm/socket.io-parser@4.2.2/node_modules/socket.io-parser/build/cjs/index.js:223:23)
    at Decoder.add (/home/user/dojo/socket.io/node_modules/.pnpm/socket.io-parser@4.2.2/node_modules/socket.io-parser/build/cjs/index.js:126:27)
    at Client.ondata (/home/user/dojo/socket.io/node_modules/.pnpm/socket.io@4.6.1/node_modules/socket.io/dist/client.js:182:26)
    at Socket.emit (node:events:513:28)
    at Socket.onPacket (/home/user/dojo/socket.io/node_modules/.pnpm/engine.io@6.4.1/node_modules/engine.io/build/socket.js:116:22)
    at Polling.emit (node:events:513:28)
    at Polling.onPacket (/home/user/dojo/socket.io/node_modules/.pnpm/engine.io@6.4.1/node_modules/engine.io/build/transport.js:92:14)
    at callback (/home/user/dojo/socket.io/node_modules/.pnpm/engine.io@6.4.1/node_modules/engine.io/build/transports/polling.js:166:18)
    at Array.forEach (<anonymous>)
    at Polling.onData (/home/user/dojo/socket.io/node_modules/.pnpm/engine.io@6.4.1/node_modules/engine.io/build/transports/polling.js:172:45)

Client code

const Manager = require("socket.io-client").Manager;
const manager = new Manager("http://127.0.0.1:3000");
const socket = manager.socket("/");

socket.on("connect", () => {
  console.log("Connected to /");
});

// `two` instead of `/two`
const socket2 = manager.socket("two");
socket2.on("connect", () => {
  console.log("Connected to `two`");
});

Server code

const express = require("express");
const app = express();
const http = require("http");
const server = http.createServer(app);
const { Server } = require("socket.io");
const io = new Server(server, {
  cors: {
    origin: "*",
  },
});

app.get("/", (req, res) => {
  res.send("Hello world");
});

io.on("connect", (socket) => {
  socket.on("error", (err) => {
    console.error("Error happened on /: " + err.stack);
  });
  console.log("Connection to / namespace");
});

io.of("two", (socket) => {
  socket.on("error", (err) => {
    console.error("Error happened on `two`: " + err.stack);
  });
  console.log("Connection to `two` namespace");
});

server.listen(3000, "127.0.0.1", () => {
  console.log("Listening on http://127.0.0.1:3000");
});

New behaviour

Slash automatically gets inserted as a prefix inside the Manager.socket function.

[Ionys320]

I guess nsp can't be another thing than a string, so it sounds good!