ISO 8601 dates: do not require time zone for date-only values

[newyankeecodeshop]

The new ISO parsing logic from issue #552 includes support for date-only forms of the string, but it requires those strings to have a time zone. The RFC for internet timestamps (http://www.ietf.org/rfc/rfc3339.txt) indicates that time zone designators should only be present when there is a time component. (See section 5.6)

ISO8601Utils.parse() should be modified to accept a date-only string (YYYY-MM-DD) with no time zone and return the value as a date corresponding to midnight on that day in the local (default) time zone.