Change unintended compile-time polymorphism to dynamic

equals(ParserIntItem&) has a different signature than equals(ParserItem&),
thus the former method does *not* overload the latter. Virtual just means
then only means that you have created a *new* entry in the v-table. If
you call equals through a pointer/reference to the base class ParserItem,
the defined method in the derived class is not called, and we miss out the
test for the equal default value.

Instead, we should take an argument of the base type and use a dynamic
cast to the derived type. If this downcast fails, then they are not equal;
otherwise we have gotten ourselves a pointer to get the properly typed
default value.

We must use pointers instead of references; if we cast to a reference,
a bad_cast exception is thrown if the expression is not in the proper
type hierarchy.