refine: the server-side http Request Body is always non-nil

[shvc]

    // For server requests, the Request Body is always non-nil
    // but will return EOF immediately when no body is present.
    // The Server will close the request body. The ServeHTTP
    // Handler does not need to.
    //
    // Body must allow Read to be called concurrently with Close.
    // In particular, calling Close should unblock a Read waiting
    // for input.
    Body io.ReadCloser

per net/htp/request.go, server-side http request body is always no-nil we should use http.NoBody instead nil here

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[dashpole]

You will need to sign the CLA before I can merge this.