PreSerialization Event not handled if the value is not object

[y1o1]

If the type of property is not object, "pre_serialization" event is not dispatched.
In other words, the method "SerializationContext::isVisiting" throw an exception before handle event listener, iff the value is not an object.

This is too limited i think.

In my case, I created Hash Type, which can be either "String hash" or "Hashable instance".
I want to serialize without replacement of the object.

[goetas]

see #610