The SerializationProxy uses this descriptor to determine which
fields and properties should be excluded from an object graph
on an instance-by-instance basis. By default, all public instance
variables and properties will be serialized.
If excludes need to be specified for a complex child property,
the property name is added to this dynamic descriptor and its value
set to another descriptor with its own set of excludes.
The absence of excludes implies default serialization. The absence of
a child property implies default serialization.