Websockets for Public topics not providing sign fails.

[Matthiaas]

This should be fixable by doing this.

--- a/lib/websockets.js
+++ b/lib/websockets.js
@@ -53,7 +53,10 @@ Sockets.initSocket = async function(params, eventHandler) {
     if ( !params.sign ) params.sign = false;
     if ( !params.endpoint ) params.endpoint = false;
     let [topic, endpoint, type] = Sockets.topics( params.topic, params.symbols, params.endpoint, params.sign )
-    let sign = this.sign('/api/v1/bullet-private', {}, 'POST')
+    var sign = null;
+    if (params.sign) {
+      sign = this.sign('/api/v1/bullet-private', {}, 'POST')
+    }
     let websocket = await getSocketEndpoint(type, this.baseURL, this.environment, sign)
     let ws = new WebSocket(websocket)
     Sockets.ws[topic] = ws