[fetch] Cache mode: partial content (#38)

@mayhemer it's not entirely clear to me how to deal with partial content. Does us handling that mean we also pay special attention to a 206 response? What's the logic here, if we notice we have a partial cache entry, we set `If-Range`, then if there's a 206 response, we combine some stuff and hand out the combined response as a 200 to content? Does this only happen for media elements basically?

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https://github.com/whatwg/fetch/issues/38