Fixed redefinition of special props in Object.defineProperty().

Fixed redefinition of special props in Object.defineProperty().

Previously, when NJS_PROPERTY_HANDLER property was updated it might be
left in inconsistent state.  Namely, prop->type was left unchanged, but
prop->value did not have an expected property handler.  As a result
consecutive reference to the property may result in a segment violation.

The fix is to update the prop->type during redefinition.

This closes #504 issue on Github.