Docs saved to CBL should not contain any "_"-prefixed properties

[snej]

Don't put properties like _id, _deleted, etc. into documents. CBL 1.x would reject them as invalid data; CBL 2 treats them like any other properties, but if you use names that conflict with ones used by SG's REST API, it will cause undefined behavior when SG tries to add those properties in its JSON responses.

mobile-testkit/CBLClient/Database.py

Line 238 in 38ed6b2

doc_body["_id"] = doc_id