message: feat(typeahead): Allow typeahead to cancel ongoing requests
  fixes
  * Allow typeahead to cancel ongoing requests
  Allow typeahead to cancel ongoing requests when new request is made.
  feat(typeahead): add possibility of canceling ongoing requests when a new request is sent
  * fix(typeahead): fix switchMap import
diff: @@ -18,6 +18,7 @@ import 'rxjs/add/observable/from';
  import 'rxjs/add/operator/debounceTime';
  import 'rxjs/add/operator/filter';
  import 'rxjs/add/operator/map';
  +import 'rxjs/add/operator/switchMap';
  import 'rxjs/add/operator/mergeMap';
  import 'rxjs/add/operator/toArray';
  @@ -343,7 +344,7 @@ export class TypeaheadDirective implements OnInit, OnDestroy {
  this._subscriptions.push(
  this.keyUpEventEmitter
  .debounceTime(this.typeaheadWaitMs)
  - .mergeMap(() => this.typeahead)
  + .switchMap(() => this.typeahead)
  .subscribe((matches: any[]) => {
  this.finalizeAsyncCall(matches);
  })
  