Try to testing an http-api-service with HttpClientTestingModule but service always returning "undefined"

I'm trying to test my angular api-service with the angular HttpClientTestingModule. I read several tutorials how to use the HttpClientTestingModule. But if I'm using this, I get a undefinde response.

Here is the service I'm trying to test

import { HttpClient } from "@angular/common/http";
import { Injectable } from "@angular/core";
import * as log from "loglevel";
import { Observable } from "rxjs";
import { map } from "rxjs/operators";
import { itemHistoryURL } from "../../../../urls";
import { IHistoryObject } from "../item-history-service/item-history.service";

@Injectable({
  providedIn: "root"
})
export class ItemHistoryApiService {
  readonly default_headers = {
    "Access-Control-Allow-Origin": "*",
    "Access-Control-Allow-Methods":
      "Origin, X-Requested-With, Content-Type, Accept",
    "Content-Type": "application/json"
  };

  constructor(public http: HttpClient) {}

  public fetchItemHistory(itemId: string, itemType: string): Observable<any> {
    log.debug(
      "Trying to fetch the complete history for item in API Service",
      itemId
    );
    return this.http
      .get(itemHistoryURL + "/itemhistory/" + itemType + "/" + itemId, {
        headers: this.default_headers
      })
      .pipe(
        map((response: any) => {
          const historyList: IHistoryObject[] = response.data;
          return historyList;
        })
      );
  }

and this is my testing-class

import { TestBed } from "@angular/core/testing";

import {
  HttpClientTestingModule,
  HttpTestingController
} from "@angular/common/http/testing";
import { itemHistoryURL } from "../../../../urls";
import { ItemHistoryApiService } from "../item-history-api-service/item-history-api.service";

describe("ItemHistoryService", () => {
  let httpTestingController: HttpTestingController;
  let service: ItemHistoryApiService;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [HttpClientTestingModule],
      providers: [ItemHistoryApiService]
    });

    httpTestingController = TestBed.get(HttpTestingController);
    service = TestBed.get(ItemHistoryApiService);
  });

  afterEach(() => {
    httpTestingController.verify();
  });

  it("should be created", () => {
    expect(service).toBeTruthy();
  });

  fit("should be fetch all HistoryData for an item", () => {
    const dummyHistoryList = [
      {
        changedBy: "Jens",
        changedAt: "15667867868568",
        changes: [
          {
            changeType: "ValueChange",
            property: "eventName",
            propertyChangeType: "PROPERTY_VALUE_CHANGED",
            entryChanges: [
              {
                entryChangeType: "",
                key: "",
                leftValue: "",
                rightValue: ""
              }
            ],
            left: "ItemCreated",
            right: "ItemPayloadChanged"
          },
          {
            changeType: "ValueChange",
            property: "costCenter",
            propertyChangeType: "PROPERTY_VALUE_CHANGED",
            entryChanges: [
              {
                entryChangeType: "",
                key: "",
                leftValue: "",
                rightValue: ""
              }
            ],
            left: "44534",
            right: "FCX_GW"
          }
        ]
      }
    ];
    service.fetchItemHistory("test", "test").subscribe(data => {
      console.log(data);
      expect(data).toEqual(dummyHistoryList);
    }, fail);

    const req = httpTestingController.expectOne(
      itemHistoryURL + "/itemhistory/" + "test" + "/" + "test"
    );
    expect(req.request.method).toBe("GET");
    req.flush(dummyHistoryList);
  });
});

The console.log(data) in the test-class always returning undefined instead of my testing-response-object.

I'm searching a lot where I have a misstake in the code but I can't finde the problem.

Can anyone help me why I'm getting undefined instead of the test-object as respone?