internal/number: allow negative zero

This is consistent with fmt

Change-Id: I7913f127e3b9d361f34857c4db0062f61d6e72c3
Reviewed-on: https://go-review.googlesource.com/46471
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed