Single broadcast API mangles JSON bodies that aren't plain objects #2378

TL;DR:
POST /api/broadcast/:topic/events/:event works out the payload type from keys in the parsed body instead of from Content-Type, so two kinds of JSON body come out wrong:

• A body that isn't an object ([1,2,3], "hello", 42) returns 202, but subscribers get {"_json": [1,2,3]}. That's how Plug wraps non-object JSON, and it's passed straight through.
• An object with a _binary key is treated as a binary payload: v2 subscribers get a binary frame containing only the _binary value, and v1 subscribers get nothing.

curl -X POST "$REALTIME_URL/api/broadcast/room/events/msg" \
  -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" \
  -d '[1,2,3]'
# subscribers receive {"_json":[1,2,3]}

curl -X POST "$REALTIME_URL/api/broadcast/room/events/msg" \
  -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" \
  -d '{"_binary":"x","other":1}'
# v2 subscribers get a binary "x", v1 subscribers get nothing

#1864 says the content type decides the payload type, and the schema describes the body as a JSON object, so non-object bodies should probably get a 422.