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const oldRequest = new Request('https://github.com/mdn/content/issues/12959',
{ headers: { 'From': 'webmaster@example.org'}});
oldRequest.headers.get("From"); // "webmaster@example.org"
const newRequest = new Request(oldRequest,
{ headers: { 'From': 'developer@example.org'}});
newRequest.headers.get('From') // "developer@example.org"