Implement default values and validation for status codes

[wheresrhys]

To comply with https://fetch.spec.whatwg.org/#dom-response, and equivalent to these 2 PRs on node-fetch
node-fetch/node-fetch#135
node-fetch/node-fetch#132

I'm happy to submit a PR if it's approved in principle

[mislav]

I'm fine of setting default HTTP status to 200 and text to "OK". This will be easy to implement. However, I don't think it's worth our time to implement the validation that the status code must be between 200–599. Our library should stay simple and doesn't need to conform to 100% of the spec— we aim to implement just the parts that will actually be useful to people in development.