requestInit.headers do not work

[liheng314]

Describe the bug
requestInit.headers do not work because requestInit.headers cannot be spread (...) when its type is Headers.
code like below may be fixed.

return new Headers(
      { ...headers, ...this._requestInit?.headers }
    );

To Reproduce
Steps to reproduce the behavior:

const headers = new Headers({ "Content-Type": "application/json" });
console.log({ ...headers });
// output is {}