title: Mercure & Event Source Polyfill :Error : No activity within 45000 milliseconds. 2 chars received. Reconnecting
body: <p>I have a symfony / api PLatform server. I added Mercure to send SSE to API clients. On my client side I subscribe using event Source Polyfill.</p>
  <p>I manage to receive the messages, but I have the following error :</p>
  <p><em>Error: No activity within 45000 milliseconds. 2 chars received. Reconnecting.</em></p>
  <p>I read that a heartbeat was needed to kieep connection alive and avoid this error.</p>
  <p>I then added the heartbeat instruction to the hub command line :</p>
  <pre><code>SERVER_NAME=:3000 MERCURE_PUBLISHER_JWT_KEY='toto' MERCURE_SUBSCRIBER_JWT_KEY=$(cat ../config/jwt/public.pem) MERCURE_SUBSCRIBER_JWT_ALG=RS256 HEARTBEAT_INTERVAL=15s READ_TIMEOUT=2m ./mercure run -config Caddyfile.dev
  </code></pre>
  <p>but nothing changed. I still have this error.</p>
  <p>What did I miss?</p>
  <p>thank you for your help</p>