The internally stored date value was getting out of sync with the real date in
the control itself when SetValue() was called with a date out of the currently
set range. This resulted in an assert failure and other unpleasantness later.
Fix the bug and add a unit test checking for it (and also with some other
basic wxDatePickerCtrl checks).
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67990 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
