GenericValue::Accept: force copy flag to bool (closes #5)

This should suppress the (harmless) warning C4800 on MSVC.
Quote from http://msdn.microsoft.com/en-us/library/b6801kcy.aspx

  "If you cannot rewrite the expression to use type bool, then you can
   add "!=0" to the expression, which gives the expression type bool.
   Casting the expression to type bool will not disable the warning,
   which is by design."