[fetch] 204 and json (#113)

When a Response is `204` and `response.json()` is called an `unexpected end of input` error is thrown. 
As the response is already stating that there is no JSON trying to parse it feels wrong. Would it make more sense in this case to either:
a) resolve with an empty value 
b) throw a more informative error e.g. `Cannot parse a response with no content as json`

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/113