[Miniflare 2] Fix body.cancel() throwing undefined

[jahands]

We noticed while writing a test for code that uses the Cache API that body.cancel() existed, but threw an error complaining that the underlying reader was undefined.

This ensures that reader is set before calling cancel(). I have included a test which failed before but works after the change.

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Latest commit: 828125c

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[mrbbot]

Thanks for fixing this! 🎉