HTTP-network-or-cache fetch request cloning

[annevk]

Thanks to @yutakahirano I spotted some weird behavior about how we deal with request cloning in the network. It sets stream to null rather than source.

But maybe we should just use clone as we also do for service workers and mention that implementers can avoid teeing due to the source null checks.

[annevk]

@yutakahirano I blamed this and it seems we did settle on this text somehow, but I guess we were both confused? 0cef38e 😊

I'll create a PR.