[fetch] " [SameObject] readonly attribute ReadableStream..." (#143)

https://fetch.spec.whatwg.org/#response

>  [SameObject] readonly attribute ReadableStream body;

As discussed @TPAC, I think that we need to tee the stream (i.e., clone it) when accessing the body, which suggests that this not use `[SameObject]` and rather that it be a method.

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https://github.com/whatwg/fetch/issues/143