Non-null response body check is a no-op

[annevk]

@yutakahirano I think with the integration of streams https://fetch.spec.whatwg.org/#concept-http-network-fetch became a little strange.

In particular, step 14 checks if response's body is non-null, but step 9 always sets it to a value. Also, step 14 sets some state on response's body to some other state that's also supposedly on response's body.

This problem stems from the fact that previously response was just an abstraction on top of HTTP, but now that works less well.

I guess the way to address it is to say that response is a hybrid between the response concept and HTTP's way of defining a response. Seems hard to solve until the layering improves.