A holder for a RequestCondition useful when the type of the request
condition is not known ahead of time, e.g. custom condition. Since this
class is also an implementation of RequestCondition, effectively it
decorates the held request condition and allows it to be combined and compared
with other request conditions in a type and null safe way.
When two RequestConditionHolder instances are combined or compared
with each other, it is expected the conditions they hold are of the same type.
If they are not, a ClassCastException is raised.