Do not replace the sign in front of a number with a space if both
f.space and f.plus are both specified for number formatting.
This was already the case for integers but not for floats
and complex numbers.

Updates: #14543.

Change-Id: I07ddeb505003db84a8a7d2c743dc19fc427a00bd
Reviewed-on: https://go-review.googlesource.com/19974
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>