Re: [fetch] Introduce Response.body. (#118) (#128)

>     <span title=concept-Headers-guard>guard</span> is "<code title>immutable</code>".
> +
> +   <li><p>If <var>response</var>'s status is a <span>null body status</span>, resolve <var>p</var>

Without the early-exit, `res` has an empty associated readable stream instead of null. I stopped using null body status here and check `response`'s body's nullity instead.

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