The `Set-Cookie` header is an exceptional case where multiple values are allowed, and should not be joined into a single header. Because of its browser-focussed origins (where set-cookie is not visible), `fetch()` does not have a clean API for this. Instead we have to access the `raw()` data.
This fixes various authentication-related issues when developing via the Ember CLI proxy.
node-fetch is now a ES module, so it has to either imported with `import/from` syntax (which can't be used in addon's index.js) or using the dynamic `import()`
