Don't silently ignore referrer

[HerrSpace]

As described in the Readme of the node-fetch repo 'referrer' is currently not implemented. The user is also not warned about its usage either though.

Reproduction

Steps to reproduce the behavior:

Run this in node:

const fetch = require('node-fetch');
var HttpProxyAgent = require('http-proxy-agent');

let url = "http://127.0.0.1"
let promise = fetch(url, {
  agent: new HttpProxyAgent('http://127.0.0.1:8080'),
  method: "GET", // POST, PUT, DELETE, etc.
  referrer: url,
});
promise
    .then(data => data.text())
    .then(text => console.log(text))

Expected behavior

1. Preferably set the Referer in the HTTP request
2. Alternatively emit a warning that 'referrer' was ignored.

Screenshots

Request in burp, missing Referer:

Your Environment

software	version
http-proxy-agent	4.0.1
node-fetch	2.6.1
node	v14.6.0
npm	6.14.7