Fixes #1384

Specification: https://fetch.spec.whatwg.org/#dom-headers-getsetcookie

Why is this needed since the browsers doesn't expose Set-Cookie? React Native is using this polyfill to provide fetch, and it does expose the Set-Cookie header.

An alternative implementation would be to store the set-cookie headers separately from the map variable. Please let me know if you would prefer that, and I'll update the code.