refactor: remove binary handling for the polling transport

Since Engine.IO v4, the binary payloads are always encoded as base64
with the polling transport.

See https://github.com/socketio/engine.io-protocol#difference-between-v3-and-v4

lib/transports/polling.js

@@ -109,12 +109,10 @@ class Polling extends Transport {
       return;
     }
 
-    const isBinary = "application/octet-stream" === req.headers["content-type"];
-
     this.dataReq = req;
     this.dataRes = res;
 
-    let chunks = isBinary ? Buffer.concat([]) : "";
+    let chunks = "";
     const self = this;
 
     function cleanup() {
@@ -131,16 +129,11 @@ class Polling extends Transport {
 
     function onData(data) {
       let contentLength;
-      if (isBinary) {
-        chunks = Buffer.concat([chunks, data]);
-        contentLength = chunks.length;
-      } else {
-        chunks += data;
-        contentLength = Buffer.byteLength(chunks);
-      }
+      chunks += data;
+      contentLength = Buffer.byteLength(chunks);
 
       if (contentLength > self.maxHttpBufferSize) {
-        chunks = isBinary ? Buffer.concat([]) : "";
+        chunks = "";
         req.connection.destroy();
       }
     }
@@ -161,7 +154,7 @@ class Polling extends Transport {
     }
 
     req.on("close", onClose);
-    if (!isBinary) req.setEncoding("utf8");
+    req.setEncoding("utf8");
     req.on("data", onData);
     req.on("end", onEnd);
   }