fix(testing): MockResponse404 is not ok

[luchsamapparat]

MockResponse404 extends from MockResponse which sets ok to true by default. However, ok should only be true, if the status code is within the 2xx range.

Alternatively, we could change the ok property of MockResponse to a getter:

export class MockResponse {
  get ok() {
    return this.status >= 200 && this.status <= 299
  }
}

However, by doing that ok would be readonly and that is probably a breaking change. 🤔

[manucorporat]

Thanks for the PR!