feat(AjaxObservable): add "abort" event listener

[olessavluk]

Closes #4251

Description:
This PR add support for XHR abort event.

Reference implementations:

• kwonoj/oxid@e783f19#diff-373902f1034fa6a981a40e97e16bee0fR253
• Fixing #537 Rejecting promise if request is cancelled by the browser axios/axios#1399

Code example:

import { ajax } from "rxjs/ajax";

const log = prefix => ({
  next: e => console.log(prefix, e),
  error: e => console.log(prefix, e),
  complete: () => console.log(prefix, "complete")
});

const request$ = ajax({
  url: "https://httpbin.org/delay/10",
  progressSubscriber: log("progress: ")
});
const r = request$.subscribe(log("request: "));

// just to simulate Chrome behavior when it aborts your request, 
// no one should do this is his code
r._subscriptions[0].xhr.abort();

Current behavior: will do nothing, just hang
With this patch: error will be thrown, and ProgressEvent with type: "abort" (see screenshot)

Browser sleep/wake behavior:

• Chrome, as described in AjaxObservable: does not error when aborted #4251 it will abort() pending requests when going to sleep
• Safari, Firefox does not abort requests
  (tested on OS X)

Related issue (if exists): Closes #4251

[ci-reporter]

The build is failing

✨ Good work on this PR so far! ✨ Unfortunately, the Travis CI build is failing as of 720a7f7. Here's the output:

npm test

> @reactivex/rxjs@6.4.0 test /home/travis/build/ReactiveX/rxjs
> cross-env TS_NODE_PROJECT=spec/tsconfig.json mocha --opts spec/support/default.opts "spec/**/*-spec.ts"



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  2606 passing (4s)
  4 pending
  1 failing

  1) ajax
       should abort unfinished request when unsubscribe:

      AssertionError: expected '/flibbertyJibbet' to equal '/url'
      + expected - actual

      -/flibbertyJibbet
      +/url
      
      at spec/observables/dom/ajax-spec.ts:338:28
      at Context.modified (spec/helpers/testScheduler-ui.ts:206:13)

I'm sure you can fix it! If you need help, don't hesitate to ask a maintainer of the project!

---

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[coveralls]

Pull Request Test Coverage Report for Build 8129

• 9 of 11 (81.82%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 96.793%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/internal/observable/dom/AjaxObservable.ts	9	11	81.82%

Totals
Change from base Build 8098:	-0.01%
Covered Lines:	5252
Relevant Lines:	5426

---

💛 - Coveralls

[benlesh]

What is the use case for this? No one should be doing this: r._subscriptions[0].xhr.abort();, as _subscriptions is an implementation detail, and could change in any given version.

[olessavluk]

Hi @benlesh
Yes, but I am running _subscriptions[0].xhr.abort(); just to emulate Chrome behavior when you put computer to sleep with pending request (as described in #4251).

[olessavluk]

@kwonoj since you have implemented the same feature in oxid
could you please take a look at this PR?

[olessavluk]

Hi @benlesh
Have you been able to review the proposed changes? Are there any concerns regarding the idea or implementation?
I have been using a fork with this fix for quite some time and no issues so far. So can it be merged to official version?

[benlesh]

Closing in favor of #6041

Sorry for the long wait. Better late than never. :)