Stop circular dependencies causing infinite recursion.

[nilportugues]

Typical example, Doctrine Proxy classes, are causing these, but any kind of circular references should stopped by placing an id (works for Doctrine, not random objects with this issue), or creating a secondary structure to hold the references to these objects.

[nilportugues]

https://github.com/nilportugues/php-serializer/blob/master/src/Serializer.php#L555 is where to patch