For some reason in my original version I was sending both
content and data to the client for submessage events,
where data === JSON.parse(content). There's no reason
to not just let the client parse it, since the client
already does it for data that comes on the original
message, and since we might eventually have non-JSON
payloads.
The server still continues to validate that the payload
is JSON, and the client will blueslip if the server
regressses and sends bad JSON for some reason.