fix: Fetch handler hacks for Mirage

[gitKrystan]

Description

• Allow store.request to be intercepted by Mirage
  • This is necessary because Mirage doesn't mock fetch until after EmberData closes over fetch.
• Don't cloneResponse for Mirage mock responses
  • This is necessary because Mirage does not implement Response.body, which is required for cloneResponse. Since cloneResponse only exists because updating the headers on a real Response isn't allowed (but Mirage also--conveniently--doesn't implement this limitation), we can just update the headers on the Mirage mock response instead.

Notes for the release