Makes zero padding of NaN and infinities consistent
by using spaces instead of zeroes to pad NaN.
Adds more tests for NaN formatting.

Fixes #14421

Change-Id: Ia20f8e878cc81ac72a744ec10d65e84b94e09c6a
Reviewed-on: https://go-review.googlesource.com/19723Reviewed-by: Rob Pike <r@golang.org>