Reset IPv6 exit-policy summary in details status.

Fixes #22030.

CHANGELOG.md

+# Changes in version 4.0-1.?.? - 2017-0?-??
+
+ * Medium changes
+   - Reset IPv6 exit-policy summary in details status if a newer
+     server descriptor doesn't contain such a summary anymore.
+
+
 # Changes in version 4.0-1.3.0 - 2017-08-04
 
  * Medium changes

src/main/java/org/torproject/onionoo/updater/NodeDetailsStatusUpdater.java

@@ -196,6 +196,8 @@ public class NodeDetailsStatusUpdater implements DescriptorListener,
       exitPolicyV6Summary.put(descriptor.getIpv6DefaultPolicy(),
           portsOrPortRanges);
       detailsStatus.setExitPolicyV6Summary(exitPolicyV6Summary);
+    } else {
+      detailsStatus.setExitPolicyV6Summary(null);
     }
     detailsStatus.setHibernating(descriptor.isHibernating() ? true :
         null);