refactor: Minor changes based on PR review

Author: vakrilov

Diff for: nativescript-angular/http-client/http-client.module.ts

@@ -9,8 +9,6 @@ global.__extends = cachedExtends;
 import { NSFileSystem } from "../file-system/ns-file-system";
 import { NsHttpBackEnd } from "./ns-http-backend";
 
-export { NsHttpBackEnd } from "./ns-http-backend";
-
 @NgModule({
     providers: [
         NSFileSystem,

Diff for: nativescript-angular/http-client/index.ts

@@ -1 +1,2 @@
 export * from "./http-client.module";
+export * from "./ns-http-backend";

Diff for: ng-sample/app/examples/http-client/http-client-test.ts

@@ -3,7 +3,7 @@ import {
     HttpClient, HTTP_INTERCEPTORS, HttpEventType, HttpErrorResponse,
     HttpEvent, HttpInterceptor, HttpHandler, HttpRequest
 } from "@angular/common/http";
-import { Observable } from "rxjs/observable";
+import { Observable } from "rxjs/Observable";
 import "rxjs/add/operator/do";
 
 @Injectable()