[release-branch.go1.3] fmt: fix signs when zero padding.

««« CL 103480043 / 777dd5a434db
fmt: fix signs when zero padding.
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
»»»

LGTM=r, rsc
R=r, rsc
CC=golang-codereviews
https://golang.org/cl/110040043
2 files changed