Resetting the field that marks the filter exported as a context attribute.

jetty-websocket/websocket-server/src/main/java/org/eclipse/jetty/websocket/server/WebSocketUpgradeFilter.java

@@ -148,6 +148,7 @@ public class WebSocketUpgradeFilter extends AbstractLifeCycle implements Filter,
     {
         try
         {
+            alreadySetToAttribute = false;
             configuration.stop();
         }
         catch (Exception e)