Fixed: CPDatePicker takes key code CPReturnKeyCode all the time

Previously, the CPDatePicker took the key code CPReturnKeyCode all the time. It means, the key event wasn't redistributed to the other component, for example a defaultButton in a window.

Now instead of returning YES, it returns [super performKeyEquivalent:anEvent]
Merge pull request #2279 from Dogild/CPDatePickerEnter

Fixed: CPDatePicker takes key code CPReturnKeyCode all the time