upcastable

https://github.com/abicky/upcastable

Duck typing sometimes results in `NoMethodError` unexpectedly by calling methods some classes don't have even if the code pass a test using other classes which have the methods. We can avoid such situations by upcasting. All we have to do is implementing methods defined in the super class or module and we don't have to care about whether or not methods defined only in some subclasses are called.