Support keepNull on insert-update

[rashtao]

Implement support for specifying keepNull option in DocumentCreateOptions, which is used by both:

• ArangoCollection.insertDocument
• ArangoCollection.insertDocuments

API doc: https://www.arangodb.com/docs/stable/http/document-working-with-documents.html#create-document

See #373

[rashtao]

Fixed in https://github.com/arangodb/arangodb-java-driver/releases/tag/v7.0.0-ALPHA.1

[rashtao]

Since vertion 7.0.0, keepNull behavior can be specified using Jackson API by:

• adding annotations on related fields, or
• registering custom serializers

see https://github.com/arangodb/arangodb-java-driver/blob/v7/docs/v7_java-reference-serialization.md.

https://github.com/arangodb/arangodb-java-driver/releases/tag/v7.0.0-RC.4

[rashtao]

Closing as fixed in version 7.0.0.

keepNull behavior can be specified using Jackson API by:

• adding annotations on related fields, or
• registering custom serializers

see https://github.com/arangodb/docs/blob/113c2351a0153171adfc74c313454af6bb5c7f08/drivers/java-reference-serialization.md.