can't restart the DefaultHTTPServer

issue:
step 1: start the server
step 2: stop the server
step 3: start the same server.
it will fail.

it seems like the stop func does not set acceptScoket to nil so it will failed to start again.

 public func start() throws {
        guard acceptSocket == nil else {
            logger.error("Server already started")
            return
        }