fix(AjaxObservable): Fix for #1921 exposed AjaxObservable unsubscript…

…ion error calling xhr.abort().

Possibly only an artifact of test utils, but after applying fix for #1921 started getting an error

that xhr.abort() in AjaxObservable's unsubscribe call wasn't a function.

src/observable/dom/AjaxObservable.ts

@@ -361,7 +361,7 @@ export class AjaxSubscriber<T> extends Subscriber<Event> {
 
   unsubscribe() {
     const { done, xhr } = this;
-    if (!done && xhr && xhr.readyState !== 4) {
+    if (!done && xhr && xhr.readyState !== 4 && typeof xhr.abort === 'function') {
       xhr.abort();
     }
     super.unsubscribe();