feat: add support for the "wsPreEncoded" writing option

A few notes:

- only the first element is pre-encoded, because the other elements are
buffers and will be sent as is over the WebSocket connection

- using `socket.packet()` was unexpectedly working, we will now use
`socket.client.writeToEngine()` (see [1])

See also: socketio/engine.io@7706b12

[1]: socketio/socket.io#3775

lib/index.ts

@@ -135,8 +135,16 @@ export class Adapter extends EventEmitter {
     packet.nsp = this.nsp.name;
     const encodedPackets = this.encoder.encode(packet);
 
+    const firstPacketOpts = {
+      wsPreEncoded: "4" + encodedPackets[0], // "4" being the "message" packet type in Engine.IO
+      ...packetOpts
+    };
+
     this.apply(opts, socket => {
-      socket.packet(encodedPackets, packetOpts);
+      socket.client.writeToEngine(encodedPackets[0], firstPacketOpts);
+      for (let i = 1; i < encodedPackets.length; i++) {
+        socket.client.writeToEngine(encodedPackets[i], packetOpts);
+      }
     });
   }
 

test/index.js

@@ -65,16 +65,20 @@ describe("socket.io-adapter", () => {
         id,
         {
           id,
-          packet() {
-            ids.push(id);
+          client: {
+            writeToEngine() {
+              ids.push(id);
+            }
           }
         }
       ];
     }
     const nsp = {
       server: {
         encoder: {
-          encode() {}
+          encode() {
+            return [];
+          }
         }
       },
       sockets: new Map([socket("s1"), socket("s2"), socket("s3")])
@@ -98,16 +102,20 @@ describe("socket.io-adapter", () => {
         id,
         {
           id,
-          packet() {
-            ids.push(id);
+          client: {
+            writeToEngine() {
+              ids.push(id);
+            }
           }
         }
       ];
     }
     const nsp = {
       server: {
         encoder: {
-          encode() {}
+          encode() {
+            return [];
+          }
         }
       },
       sockets: new Map([socket("s1"), socket("s2"), socket("s3")])