bug#14545: date --iso-8601 should use colon in time zone offset

[Jordon Kalilich]

date uses an inconsistent format when the options
--iso-8601=[hours|minutes|seconds|ns] are specified. Example:

$ date --iso-8601=seconds
2013-06-02T20:07:03-0700

The output mostly follows the ISO 8601:2004 extended format but fails to include
a colon in the time zone offset, as required by section 4.3.2 "Complete
representations" and section 4.3.3 "Representations other than complete."

In this example, date should print the date as 2013-06-02T20:07:03-07:00 and
should likewise include a colon in the time zone offset with the "hours,"
"minutes," and "ns" options.

This is based on my use of date 8.20, as packaged with Ubuntu 13.04. I didn't
see anything about this in the shortlog of changes in coreutils 8.21.

Jordon Kalilich