Handle invalid dates properly for QDataStream versions < 5.

Handle invalid dates properly for QDataStream versions < 5.

In Qt 5, when streaming an invalid QDate using a QDataStream version
earlier than Qt_5_0, QDate.jd is written and read as 0, which is an
invalid julian day for Qt versions earlier than 5.0. For Qt 5.0
however, 0 is a valid julian day, so when comparing a deserialised
invalid date (read using a QDataStream version < Qt_5_0) against a
default-constructed invalid date, they won't compare equal when they
should.

Task-number: QTBUG-26989

Change-Id: Ia76df493471f3b068c7d7187be20e3178eff2cc7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>