add expectations for numeric status codes

[wheresrhys]

This implements another bit of conformance with the Response spec. https://fetch.spec.whatwg.org/#dom-response.

[coveralls]

Coverage remained the same at 100.0% when pulling f750b64 on wheresrhys:numeric-status into 952aa2a on bitinn:master.

[bitinn]

I decide to close this PR, I know you are doing the right thing, but also note that status is not completely under developer control, as underlying Node.js http.request might not normalize the status code between 200-599...

It makes sense for us to throw RangeError on new Response() (programming error), but not quite right to reject with RangeError when response is outside of 200-599 range (not a programming error, which should use custom FetchError)...

Anyway. thx for pointing this out :)