Kzl 791 fix exception when publish msg with rest api

[etrousset]

What does this PR do?

Fix exception when subscribed to a channel in which someone publish a message using REST API

See https://jira.kaliop.net/browse/KZL-791

How should this be manually tested?

Use the following client 👍

const Kuzzle  = require('kuzzle-sdk').Kuzzle;


const kuzzle = new Kuzzle('websocket', {host: 'localhost'});

async function run() {
 await kuzzle.connect();
 const roomID = await kuzzle.realtime.subscribe('obix', 'firmwares', {}, (r) => {
   console.log(r);
 })
 
 await kuzzle.realtime.publish('obix', 'firmware', {msg: 'hello world'})
}

run()

Use rest API to publish a message:

POST http://localhost:7512/obix/firmwares/_publish HTTP/1.1
Content-Type: application/json

{
  "msg" : "Heelo REST API"
}

=> No more exception

[codecov-io]

Codecov Report

Merging #333 into 6-dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##            6-dev     #333   +/-   ##
=======================================
  Coverage   97.49%   97.49%           
=======================================
  Files          28       28           
  Lines        1395     1395           
=======================================
  Hits         1360     1360           
  Misses         35       35

Impacted Files	Coverage Δ
src/controllers/realtime/room.js	100% <100%> (ø)	⬆️

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