feat(typeahead): Allow typeahead to cancel ongoing requests (#3865)

fixes #1626 

* 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 #1626

* fix(typeahead): fix switchMap import

src/typeahead/typeahead.directive.ts

@@ -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);
         })