Allow saving heartbeat settings from web

code/espurna/mqtt.ino

@@ -316,7 +316,9 @@ unsigned long _mqttNextMessageId() {
 #if WEB_SUPPORT
 
 bool _mqttWebSocketOnReceive(const char * key, JsonVariant& value) {
-    return (strncmp(key, "mqtt", 3) == 0);
+    if (strncmp(key, "mqtt", 3) == 0) return true;
+    if (strncmp(key, "hb", 2) == 0) return true;
+    return false;
 }
 
 void _mqttWebSocketOnSend(JsonObject& root) {