null client is not correct for fetching origin policies

[domenic]

We need to instead reuse the client from the original fetch. This ensures proper HTTP cache partitioning per whatwg/fetch#943, and probably several other good things.

This issue covers the network fetch in https://wicg.github.io/origin-policy/#update-an-origins-origin-policy. #73 covers the cache fetch in https://wicg.github.io/origin-policy/#retrieve-the-cached-origin-policy which has other problems detailed there.