Make BaseDocument serializable

[m4ttek]

Is there any specific reason why BaseDocument is not serializable? It's just a wrapper for a map type.

Caused by: java.io.NotSerializableException: com.arangodb.entity.BaseDocument
	at java.base/java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1187)
	at java.base/java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1572)
	at java.base/java.io.ObjectOutputStream.writeRecordData(ObjectOutputStream.java:1490)
	at java.base/java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1434)
	at java.base/java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1181)
	at java.base/java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:350)
	at org.apache.flink.util.InstantiationUtil.serializeObject(InstantiationUtil.java:553)
	at org.apache.flink.util.InstantiationUtil.clone(InstantiationUtil.java:624)

[rashtao]

Thanks for reporting, I have implemented this in #596