perf: add a "wsPreEncoded" writing option

This option will be used when broadcasting a packet to multiple clients,
in order to only encode the packet once.

Usage:

```js
socket.write("test", {
  wsPreEncoded: "4test"
});
```

Note: pre-encoding the content with HTTP long-polling is a bit harder,
since the concatenation of the packets is specific to each client.

lib/transports/websocket.js

@@ -76,7 +76,7 @@ class WebSocket extends Transport {
       opts.compress = packet.options.compress;
     }
 
-    this.parser.encodePacket(packet, this.supportsBinary, data => {
+    const send = data => {
       if (this.perMessageDeflate) {
         const len =
           "string" === typeof data ? Buffer.byteLength(data) : data.length;
@@ -91,7 +91,13 @@ class WebSocket extends Transport {
         if (err) return this.onError("write error", err.stack);
         this.send(packets);
       });
-    });
+    };
+
+    if (packet.options && typeof packet.options.wsPreEncoded === "string") {
+      send(packet.options.wsPreEncoded);
+    } else {
+      this.parser.encodePacket(packet, this.supportsBinary, send);
+    }
   }
 
   /**

test/server.js

@@ -2639,6 +2639,27 @@ describe("server", () => {
         });
       });
     });
+
+    describe("pre-encoded content", () => {
+      it("should use the pre-encoded content", done => {
+        engine = listen(port => {
+          client = new eioc.Socket("ws://localhost:%d".s(port), {
+            transports: ["websocket"]
+          });
+
+          engine.on("connection", conn => {
+            conn.send("test", {
+              wsPreEncoded: "4test pre-encoded"
+            });
+          });
+
+          client.on("message", msg => {
+            expect(msg).to.be("test pre-encoded");
+            done();
+          });
+        });
+      });
+    });
   });
 
   describe("packet", () => {