fix(XmppConnection.connected): check the underlying websocket

..readyState to tell if the connection is connected.

modules/xmpp/MockClasses.js

     constructor() {
         super();
         this.sentIQs = [];
+        this._proto = {
+            socket: undefined
+        };
     }
 
     /**
      * @returns {void}
      */
     simulateConnectionState(newState) {
+        if (newState === Strophe.Status.CONNECTED) {
+            this._proto.socket = {
+                readyState: WebSocket.OPEN
+            };
+        } else {
+            this._proto.socket = undefined;
+        }
         this._connectCb(newState);
     }
 

modules/xmpp/XmppConnection.js

      * @returns {boolean}
      */
     get connected() {
-        return this._status === Strophe.Status.CONNECTED || this._status === Strophe.Status.ATTACHED;
+        const websocket = this._stropheConn && this._stropheConn._proto && this._stropheConn._proto.socket;
+
+        return (this._status === Strophe.Status.CONNECTED || this._status === Strophe.Status.ATTACHED)
+            && (!this.isUsingWebSocket || (websocket && websocket.readyState === WebSocket.OPEN));
     }
 
     /**