time: Fix missing colon when formatting time zone offsets with seconds

When formatting time zone offsets with seconds using the stdISO8601Colon
and stdNumColon layouts, the colon was missing between the hour and minute
parts.

Fixes issue 8497.

LGTM=r
R=golang-codereviews, iant, gobot, r
CC=golang-codereviews
https://codereview.appspot.com/126840043

Committer: Rob Pike <r@golang.org>
A+C: Joel Stemmer (individual CLA)

Generated by a+c.

R=gobot
CC=golang-codereviews
https://codereview.appspot.com/127800043