Fix https://github.com/github/fetch/issues/657

[JakeChampion]

Fixes #657

[jungdj]

Can you please add some fallback such as an empty object?
I know this whatwg-fetch is intended to be used only in browser, however, it gives many conveniences by just "importing it" and not using it.

[jungdj]

To illustrate more, I'm suggesting to make importing 'whatwg-fetch' in nodejs environment works, while blocking the usage.

[JakeChampion]

This works in NodeJS correctly because the global this works in NodeJS.

Here the output of running it in NodeJS

❯ node -v
v14.4.0
❯ node
Welcome to Node.js v14.4.0.
Type ".help" for more information.
> var global = (function(self) {
...   return self
...   // eslint-disable-next-line no-invalid-this
... })(typeof self !== 'undefined' ? self : this)
undefined
> global
<ref *1> Object [global] {
  global: [Circular *1],
  clearInterval: [Function: clearInterval],
  clearTimeout: [Function: clearTimeout],
  setInterval: [Function: setInterval],
  setTimeout: [Function: setTimeout] {
    [Symbol(nodejs.util.promisify.custom)]: [Function (anonymous)]
  },
  queueMicrotask: [Function: queueMicrotask],
  clearImmediate: [Function: clearImmediate],
  setImmediate: [Function: setImmediate] {
    [Symbol(nodejs.util.promisify.custom)]: [Function (anonymous)]
  }
}
> 
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>