WebSocketServer cannot be started without .start() #855

Describe the bug
Before #850 you were able to start the server by using e.g. new Thread(websocketServer).start() and therefore not calling the native .start() method.
This does not work any more

To Reproduce
Steps to reproduce the behavior:

1. Start server with new Thread(webSocketServer).start()
2. Try to connect

Expected behavior
Connection is possible

Environment(please complete the following information):

• Version used: 1.4.0-SNAPSHOT

Additional context
Found in the regressions test