Testing an API call in Angular 2

I have the following 2 components and a single service that is shared by both. I need to unit test them but I'm unable to figure out how to test the dependency of the service on the following components.

//a.component.ts
import { Component, Input } from '@angular/core';
import { Http, Response } from '@angular/http';
import { SharedService } from './shared/shared.service';

@Component({
  selector: 'a',
  providers: [],
  templateUrl: './a.component.html'
})
export class AComponent {
  ax = {};

  constructor(public helperService: SharedService) {
    helperService.getFromAPI().subscribe(data => this.ax = data["people"]);
  }

}



//b.component.ts
import { Component } from '@angular/core';
import { SharedService } from './shared/shared.service';
import { Subscription }   from 'rxjs/Subscription';


@Component({
  selector: 'b',
  providers: [],
  templateUrl: './b.component.html'
})

export class BComponent {
  subscription: Subscription;
  x = '';

  constructor(public helperService: SharedService) {}

  ngOnInit() {
    this.subscription = this.helperService.c$.subscribe(
      data => {
        this.x = data;
      });
  }
}

This is the service that calls the API. The other function setC add the value to the observable upon a button click and this value is to be accessed by the BComponent.

// shared.service
import { Injectable } from '@angular/core';
import { Http, Response } from '@angular/http';
import { Subject } from 'rxjs/Subject';
import 'rxjs/add/operator/map';

@Injectable()
export class SharedService {

  private c = new Subject<string>();
  c$ = this.c.asObservable();

  constructor(
    private http: Http
  ) { }

  getFromAPI() {
    return this.http.get('url')
      .map((res: Response) => res.json());
  }

  setC(data: string) {
    this.c.next(data);
  }
}

How can I test this in Jasmine? So far, my efforts have been futile.

I've tried doing like

it('xxx', inject([SharedService], (service: SharedService) => {
    const fixture = TestBed.createComponent(AComponent);
    const app = fixture.componentInstance;

    spyOn(service, 'c$').and.callThrough;

    service.setC('Random Name');
    expect(service.c$).toHaveBeenCalled();

  }));

This fails the test with Expected spy c$ to have been called..