fix: set default protocol version to 3 (#616)

socket.io-client-swift libs version <=15.2.0, which uses protocol
version 3, do not explicitly add the EIO query parameter at transport
initialization. This omission leads the server to treat the client as
a client that supports the protocol version 4, previously set as
default, which is not correct for those versions of the client lib.

From socket.io-client-swift version v16.0.0 the EIO query parameter is
explicitly passed to specify the protocol version supported, but since
the allowEIO3 parameter aims to make the server compatible with
previous versions which in most of the cases are already used in
production and not easily upgradable, it makes more sense to default
the EIO version to 3 if not explicitly set by the client since the
newer client versions pass the EIO protocol version in query
parameters.

Related: socketio/socket.io#3794

lib/server.js

@@ -229,7 +229,7 @@ class Server extends EventEmitter {
    * @api private
    */
   async handshake(transportName, req) {
-    const protocol = req._query.EIO === "3" ? 3 : 4; // 4th revision by default
+    const protocol = req._query.EIO === "4" ? 4 : 3; // 3rd revision by default
     if (protocol === 3 && !this.opts.allowEIO3) {
       debug("unsupported protocol version");
       sendErrorMessage(

lib/transport.js

@@ -22,8 +22,8 @@ class Transport extends EventEmitter {
     super();
     this.readyState = "open";
     this.discarded = false;
-    this.protocol = req._query.EIO === "3" ? 3 : 4; // 4th revision by default
-    this.parser = this.protocol === 3 ? parser_v3 : parser_v4;
+    this.protocol = req._query.EIO === "4" ? 4 : 3; // 3rd revision by default
+    this.parser = this.protocol === 4 ? parser_v4 : parser_v3;
   }
 
   /**