fix for GetDefaultMappingFromEnumType

changed type.IsNullable() to enumType.IsNullable(). The issue I got was trying to update nullable int table field from the class having nullable Enum property. It appears that ConvertBuilder.GetDefaultMappingFromEnumType do not respect original enum type when trying to decide if the type mapping to should also be be made nullable. Please review and let me know if it makes sense for you.

Thanks!