(#2534) - use less storage in websql

This doesn't actually fix #2534 (in fact the popup still comes at exactly the same spot in the tests - around 97%), but it may have other benefits. I'll try to get some numbers on what kind of storage impact this actually has in the real world.

The idea here is if a doc only contains `{_id: "E0FC7135-9107-946B-AD68-FC67CEA4A8AE", _rev: "1-5d94ca8f4bd707979b5d13fb17ab93b4"}`, then we only really need to stringify `{}` in websql because the id/rev can be inferred.