Make Headers instantiable from native Headers

[pistvan]

Given, that I have two fetch middlewares:

// FirstMiddleware.js
export function firstMiddleware(req, next) {
  const headers = new Headers(req.headers); // native Headers type
  headers.set('X-Foo', 'bar');
  return next({ ...req, headers });
}

// SecondMiddleware.js
import { Headers } from 'node-fetch';

export function secondMiddleware(req, next) {
  const headers = new Headers(req.headers); // a polyfill type
  headers.set('X-Bar', 'bar');
  return next({ ...req, headers });
}

In SecondMiddleware, the pre-added headers are no longer kept, as the polyfill can not handle the native type, if it is available for any reason.
This PR fixes this issue.