Fix timezone issues in #attribute_to_time/#to_iso8601_date

[denisdefreyne]

Summary

• Make #attribute_to_time convert Date to UTC (rather than local) Time
• Make #to_iso8601_date return UTC representation

Detailed description

#to_iso8601_date currently deliberately does not convert to UTC, because the date is supposed to be timezone-agnostic.

For example, if I were to write 2016-10-22, then I intend that specific day, no matter where I am, and what time it is. Converting this date to UTC first could change that date to 2016-10-21 or 2016-10-23, which is not what’s intended.

Unfortunately, this does not hold up when I write a Time (e.g. 2016-10-22 10:15:00) rather than a Date and then run it through #to_iso8601_date.

In order to make both cases work, this PR converts Date to UTC Times rather than local Times, and then makes #to_iso8601_date convert to UTC.

This PR also uncovered an interesting case in the XML sitemap helper (arguably a bug), which is now also fixed.

See also

• Fix generated Atom tags being timezone-dependent #862 (original fix for this issue)

[denisdefreyne]

CC @jethrogb — this is the continuation of your PR that you originally opened!