JSON encode binary epilogue
Accumulate's binary encoding scheme tolerates (mostly) arbitrary attached to the end of a record. We'll call this the epilogue. The epilogue allows forwards-compatibility - if program A knows about a new field but program B does not, when A sends a record with the new field to B, B will consider the new field to be part of the epilogue and thus preserve it.
When the API encodes a record to JSON or decodes a record from JSON, it does not have any mechanism to provide an epilogue, and thus the epilogue is lost (when encoding) or cannot be provided (when decoding). This should be fixed.