Support for Range header in Service Worker

[HadrienGardeur]

There seems to be a number of problems with the range header which are problematic when either seeking through an audio track.

There are active discussions about this matter at:

• http://stackoverflow.com/questions/37612177/cannot-play-cached-audio-from-service-worker
• Handling Partial Content / 206 whatwg/fetch#144 (comment)

Might be fixed by using a polyfill for handling range in the SW itself, but not ideal: https://samdutton.github.io/samples/service-worker/prefetch-video/