Properly handle negative years when printing JS Dates to strings.

JavaScript knows a year 0. That is correctly translated into QDateTime
terms when creating a Date object, but it's not correctly translated
back when converting the JavaScript date to a string.

Task-number: QTBUG-29491
Change-Id: I46b200a144434187656d08e87f422f97523acd0e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>