Parts of Fetch API not implemented

Before submitting a new issue

• I tested using the latest version of the library, as the bug might be already fixed.
• I tested using a supported version of react native.
• I checked for possible duplicate issues, with possible answers.

Bug summary

Quickly tried this out in a fairly big project, and it failed on a few edge cases that I ran into (apparently not edge casey enough):

• Headers interface not implemented (used to do stuff like response.headers.get()), see https://developer.mozilla.org/en-US/docs/Web/API/Headers
• Response.clone not implemented, used by msw and others, see https://developer.mozilla.org/en-US/docs/Web/API/Response/clone

It's not an exhaustive list, and I'd be down to send a PR if you agree that this is useful.

Library version

0.1.3

Environment info

N/A

Steps to reproduce

1. …
2. …

Reproducible example repository

N/A

Locate the library's Fetch API implementation and examine how Headers and Response are currently exposed. Use the linked MDN behavior as the reference, then add coverage for response.headers.get() and Response.clone(); done means both reported gaps work in the supported React Native environment.