Miniflare3: Add Content-Length and FixedLengthStream to R2 get response

[gabivlj]

Before this, we would see that the following code wouldn't work:

  const res = await env.BUCKET.get(name);
  await env.BUCKET.put(name, res!.body);

As it would throw:
TypeError: Provided readable stream must have a known length (request/response body or readable half of FixedLengthStream)

[changeset-bot]

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Latest commit: 515865e

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[mrbbot]

Looks good! As part of reviewing this, I wrote a test. Rather than asking you to rewrite this, I've just committed it. 😃