Unioned cf type on RequestInit does not allow re-assignment of properties

[third774]

The unioned type introduced by PR #18 does not allow re-assignment of properties. Consider the following snippet:

// this assignment works just fine
let init: RequestInit = {
  cf: {
    cacheKey: 'foo',
  },
}

if (init.cf) {
  // but re-writing properties is prevented by the type checker
  init.cf.cacheKey = 'bar'
}

This produces the following type error:

The ideal outcome (if possible) would be to extend the RequestInit that fetch uses without obstructing the use case above while still supporting the example identified in issue #15.

Still trying to work out how to do this or if it's possible, but wanted to start a discussion around it :)

[1000hz]

Addressed via #44