message: test(delay): add test to prove that the previous behavior is composable
diff: -import { of } from 'rxjs';
  -import { delay, repeatWhen, skip, take, tap, mergeMap } from 'rxjs/operators';
  +import { of, concat } from 'rxjs';
  +import { delay, repeatWhen, skip, take, tap, mergeMap, ignoreElements } from 'rxjs/operators';
  import { TestScheduler } from 'rxjs/testing';
  import * as sinon from 'sinon';
  import { expect } from 'chai';
  @@ -186,4 +186,19 @@ describe('delay operator', () => {
  expectObservable(result).toBe(expected);
  });
  +
  + it('should be possible to delay complete by composition', () => {
  + const e1 = hot('---a--b--| ');
  + const t = time( '--| ');
  + const expected = '-----a--b--|';
  + const subs = '^ ! ';
  +
  + const result = concat(
  + e1.pipe(delay(t, rxTestScheduler)),
  + of(undefined).pipe(delay(t, rxTestScheduler), ignoreElements()),
  + );
  +
  + expectObservable(result).toBe(expected);
  + expectSubscriptions(e1.subscriptions).toBe(subs);
  + });
  });
  