Regression in v5.0.8+ fakeServer when $.ajax() XHR includes headers

[scottohara]

Describe the bug
In sinon@5.0.7, using jQuery's $.ajax() to fetch a non-existent URL from fakeServer returns a 404 Not Found response, as expected.

In sinon@5.0.8 and later, the same fetch now returns a 0 status.

To Reproduce

1. npm install sinon@5.0.7, and run the tests below
2. npm install sinon@5.0.8, and re-run the tests again

describe.only("404 Not Found", () => {
  let fakeServer;

  beforeEach(() => (fakeServer = sinon.fakeServer.create({respondImmediately: true})));

  it("should work without headers", done => {
    $.ajax({
      url: "/bad",
      headers: {},
      error(response) {
        response.status.should.equal(404);
        response.statusText.should.equal("Not Found");
        done();
      }
    });
  });

  it("should work with headers", done => {
    $.ajax({
      url: "/bad",
      headers: {"X-FOO": 1},
      error(response) {
        response.status.should.equal(404);
        response.statusText.should.equal("Not Found");
        done();
      }
    });
  });

  afterEach(() => fakeServer.restore());
});

Expected behavior
The specs above create a fakeServer and then attempt to fetch a non-existant URL (/bad).
The only difference between the two specs is the second includes a request header, X-FOO: 1.
The specs assert the response returned from fakeServer is a 404 Not Found

In sinon@5.0.7, both specs pass.
In sinon@5.0.8 and later, the spec with the X-FOO request header fails

Screenshots
Results from sinon@5.0.7:

Results from sinon@5.0.8:

Context (please complete the following information):

• Library version: 5.0.8+
• Environment:
• Example URL:
• Other libraries you are using: jquery@3.3.1, webpack@4.10.2, chai@4.1.2, mocha@5.2.0

[fatso83]

Probably happened in 082f8e8, going from nise@1.2.0 to nise@1.3.3. Moving this issue over to the nise subproject.

[fatso83]

This issue was moved to sinonjs/nise#51

[fatso83]

This issue was moved to sinonjs/nise#52

[fatso83]

Thanks, @scottohara!