Function appendHeader

http/unstable_message_signatures.ts:858–865

(headers: Headers, name: string, value: string)

}

function appendHeader(headers: Headers, name: string, value: string): void {
  const existing = headers.get(name);
  if (existing) {
    headers.set(name, `${existing}, ${value}`);
  } else {
    headers.set(name, value);
  }
}

// Per RFC 9421 §4, signature labels MUST be unique within an HTTP message
// across both the Signature-Input and Signature dictionaries. Appending a