XW-620 Cleanup all ThreadLocal references on context shutdown

git-svn-id: https://svn.apache.org/repos/asf/struts/struts2/trunk@759279 13f79535-47bb-0310-9956-ffa450edef68

core/src/main/java/org/apache/struts2/dispatcher/Dispatcher.java

@@ -287,6 +287,9 @@ public class Dispatcher {
         	interceptor.destroy();
         }
 
+        //cleanup action context
+        ActionContext.setContext(null);
+
         // clean up configuration
     	configurationManager.destroyConfiguration();
     	configurationManager = null;