As mentioned in #22802, only day component of layout string has space
padding(represented by one underscore before its placeholder). This
commit expands the rule for month, hour, minute and second.

Updates #22802 (maybe fixes it)

Change-Id: I886998380489862ab9a324a6774f2e4cf7124122
Reviewed-on: https://go-review.googlesource.com/78735
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>