Validation of leap seconds with TZ offset

[carlo-quinonez]

This test case fails, but should pass.

      {
        "description": "a valid iso-date-time string (leap second with timezone offset) ",
        "data": "2017-01-01T00:59:60-1",
        "valid": true
      }

According to the section on leap seconds

Further, in time zones other than "Z", the leap second point is
shifted by the zone offset (so it happens at the same instant around
the globe).