Change date format to ISO8601 on JSON and XML serialization

[garcia-jj]

At this time vraptor serializes date objects using yyyy-MM-dd HH:mm:ss.

But I think that we need to change this behaviour to use ISO8601 yyyy-MM-dd'T'HH:mm:ssZ, format defined as default date format as decribed where: http://www.w3.org/TR/NOTE-datetime.

This issue can impact in existing code, so we can postpone this issue to next major release, warning users about this in release notes.