Add compatibility test for mongodb-extended-json

[eerxuan]

Part of #25.

Category: core / data-types

Add compatibility test coverage for the mongodb-extended-json feature.

Note: a smoke test exists; comprehensive compatibility test coverage is needed.

[danielfrankcom]

May be out of scope, seems like client-side behavior.

[eerxuan]

Closing as descoped. Reason: Client-side behavior.