[fetch] consider providing a json serializer for Request and Response (#22)

Currently `JSON.stringify()` returns simply `{}` for Request and Response objects.  It might be nice to define serializers for these classes to help developers debug their code.

http://heycam.github.io/webidl/#idl-serializers

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/22