internal/number: fix rounding with DigitShift Change-Id: I52454e27295200f1afcfea471f5020257af6d13b Reviewed-on: https://go-review.googlesource.com/60590 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>