Allow package.created not include a timezone part

[roll]

Rationale

Currently, package.created is required to be RFC3339, i.e. JSONSchema's date-time format for strings. It requires to contain a timezone.

For many systems and workflows, the timezone is not applicable. For example, here is a CKAN extension data package endpoint output:

A similar problem is actual for Zenodo datapackage.json exporter and others.

[ezwelty]

@roll Could you elaborate a bit more about the rationale? The way I see it, why bother with a time if you don't know the timezone? Then you might as well just have a date.

[roll]

@ezwelty
Actually, I think for publishing dates as in DataCites are pretty enough. At the same time if data package is used internally, tz might be not needed.

Anyway let's just keep for further discussion - frictionlessdata/datapackage#933