res.body seems to the API that has shipped in latest Canary

I might be wrong - but it does seem to work for:-
https://github.com/serviceworker/offline-news-service-worker [ See here https://github.com/serviceworker/offline-news-service-worker/pull/7 ]

Found by poking around at the response object in debugger in Canary:-

![screen shot 2014-09-13 at 01 23 30](https://cloud.githubusercontent.com/assets/825088/4258873/3f974e10-3adc-11e4-92a9-9d344bcf9765.png)


// cc @jakearchibald