refactor: remove binary handling for the polling transport (bis)

The `supportsBinary` attribute and the `forceBase64` option are kept
untouched, though they are not used by the polling transport, which
now always converts the payloads to base64.

Related: socketio/socket.io-client#1391

lib/transports/polling-xhr.js

@@ -59,11 +59,7 @@ class XHR extends Polling {
    * @api private
    */
   request(opts = {}) {
-    Object.assign(
-      opts,
-      { supportsBinary: this.supportsBinary, xd: this.xd, xs: this.xs },
-      this.opts
-    );
+    Object.assign(opts, { xd: this.xd, xs: this.xs }, this.opts);
     return new Request(this.uri(), opts);
   }
 
@@ -75,11 +71,9 @@ class XHR extends Polling {
    * @api private
    */
   doWrite(data, fn) {
-    const isBinary = typeof data !== "string" && data !== undefined;
     const req = this.request({
       method: "POST",
-      data: data,
-      isBinary: isBinary
+      data: data
     });
     const self = this;
     req.on("success", fn);
@@ -122,8 +116,6 @@ class Request extends Emitter {
     this.uri = uri;
     this.async = false !== opts.async;
     this.data = undefined !== opts.data ? opts.data : null;
-    this.isBinary = opts.isBinary;
-    this.supportsBinary = opts.supportsBinary;
 
     this.create();
   }
@@ -164,17 +156,11 @@ class Request extends Emitter {
             }
           }
         }
-      } catch (e) {
-        console.log(e);
-      }
+      } catch (e) {}
 
       if ("POST" === this.method) {
         try {
-          if (this.isBinary) {
-            xhr.setRequestHeader("Content-type", "application/octet-stream");
-          } else {
-            xhr.setRequestHeader("Content-type", "text/plain;charset=UTF-8");
-          }
+          xhr.setRequestHeader("Content-type", "text/plain;charset=UTF-8");
         } catch (e) {}
       }
 
@@ -200,18 +186,6 @@ class Request extends Emitter {
         };
       } else {
         xhr.onreadystatechange = function() {
-          if (xhr.readyState === 2) {
-            try {
-              const contentType = xhr.getResponseHeader("Content-Type");
-              if (
-                (self.supportsBinary &&
-                  contentType === "application/octet-stream") ||
-                contentType === "application/octet-stream; charset=UTF-8"
-              ) {
-                xhr.responseType = "arraybuffer";
-              }
-            } catch (e) {}
-          }
           if (4 !== xhr.readyState) return;
           if (200 === xhr.status || 1223 === xhr.status) {
             self.onLoad();

lib/transports/websocket.js

@@ -27,12 +27,7 @@ class WS extends Transport {
   constructor(opts) {
     super(opts);
 
-    const forceBase64 = opts && opts.forceBase64;
-    if (forceBase64) {
-      this.supportsBinary = false;
-    }
-    // WebSockets support binary
-    this.supportsBinary = true;
+    this.supportsBinary = !opts.forceBase64;
   }
 
   /**