Ensure that NOT_FOUND resource singleton is properly deserialized

On deserialization of a NOT_FOUND resource we want to get the same
singleton instance each time.

Change-Id: Iee16671e8d176f061990eed2d26e7187fa75de39
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-httpd/src/main/java/com/google/gerrit/httpd/resources/Resource.java

@@ -44,6 +44,10 @@ public abstract class Resource implements Serializable {
     public boolean isUnchanged(long latestModifiedDate) {
       return false;
     }
+
+    protected Object readResolve() {
+      return NOT_FOUND;
+    }
   };
 
   public abstract boolean isUnchanged(long latestModifiedDate);