Bug was introduced recently. Add more tests, fix the bugs.
Suppress + sign when not required in zero padding.
Do not zero pad infinities.
All old tests still pass.
This time for sure!
Fixes #8217.

LGTM=rsc
R=golang-codereviews, dan.kortschak, rsc
CC=golang-codereviews
https://golang.org/cl/103480043