define constant for timeout result

plugins/async/src/main/java/org/apache/struts2/async/AsyncAction.java

@@ -27,6 +27,12 @@ import java.util.concurrent.Executor;
  * @since 2.6
  */
 public class AsyncAction implements Callable {
+
+    /**
+     * The action invocation was successful but did not return the result before timeout.
+     */
+    public static final String TIMEOUT = "timeout";
+
     private Callable callable;
     private Long timeout;
     private Executor executor;

plugins/async/src/main/java/org/apache/struts2/async/DefaultAsyncManager.java

@@ -132,7 +132,7 @@ public class DefaultAsyncManager implements AsyncManager, AsyncListener {
     @Override
     public void onTimeout(AsyncEvent asyncEvent) throws IOException {
         LOG.debug("Processing timeout for " + asyncEvent.getAsyncContext());
-        setAsyncActionResultAndDispatch("timeout");
+        setAsyncActionResultAndDispatch(AsyncAction.TIMEOUT);
     }
 
     @Override