Not returning proper blob

[johanbook]

This is the same as issue #105.

The Blob returned from the blob function of a mocked response is not a proper blob.

Here is a test that showcases the problem:

beforeEach(() => {
  fetchMock.mockResponse("some-data");
});

test("jest-fetch-mock can return blob", async () => {
  const res = await fetch("url");
  const blob = await res.blob();
  expect(blob).toBeInstanceOf(Blob);
});

which yields the error

    expect(received).toBeInstanceOf(expected)

    Expected constructor: Blob
    Received constructor: Blob

      16 |   const res = await fetch("url");
      17 |   const blob = await res.blob();
    > 18 |   expect(blob).toBeInstanceOf(Blob);
         |                ^
      19 | });

      at Object.<anonymous> (src/App.test.tsx:18:16)

This means in-code typechecks such as blob instanceOf Blob fails in tests.

jest-fetch-mock version: v3.0.3
Node version: v16.4.1