fix StreamingMessageHandler - null reference
fix StreamingMessageHandler - null reference

- assign value to _orderEventJsonConverter in SetAuthentication()
  _before_ it's referenced by Transmit(), fixing nullpointer;

jameschch/Lean,jameschch/Lean,jameschch/Lean,AlexCatarino/Lean,QuantConnect/Lean,AlexCatarino/Lean,JKarathiya/Lean,jameschch/Lean,JKarathiya/Lean,StefanoRaggi/Lean,AlexCatarino/Lean,QuantConnect/Lean,JKarathiya/Lean,StefanoRaggi/Lean,QuantConnect/Lean,QuantConnect/Lean,StefanoRaggi/Lean,jameschch/Lean,AlexCatarino/Lean,StefanoRaggi/Lean,StefanoRaggi/Lean,JKarathiya/Lean