refactor: merge the polling.ts and polling-xhr.ts files

In the past, there were two implementations of the HTTP long-polling
feature, one with XMLHttpRequest and one based on JSONP for ancient
browsers (IE7/IE8). The JSONP implementation has been removed in [1].

[1]: b2c7381

Author: darrachequesne

lib/transports/index.ts

@@ -1,7 +1,7 @@
-import { XHR } from "./polling-xhr.js";
+import { Polling } from "./polling.js";
 import { WS } from "./websocket.js";
 
 export const transports = {
   websocket: WS,
-  polling: XHR
+  polling: Polling
 };