[fetch-tests] Align Fetch Request tests with WebKit ones (#2549)

Adding new tests coming from WebKit:
- Checking how headers are cloned when cloning a request
- Various (good and failing) JSON parsing when calling Request.json()
- Checking that a HEAD request cannot take a body
Making some minor clean-up to some tests.


View on GitHub: https://github.com/w3c/web-platform-tests/pull/2549

Received on Monday, 1 February 2016 14:00:08 UTC