Don't set the currentSessionId to non-null unless we found the session in Redis. Fixes #25.

Author: jcoleman

src/main/java/com/radiadesign/catalina/session/RedisSessionManager.java

@@ -344,6 +344,9 @@ public Session findSession(String id) throws IOException {
 
       if (session != null) {
         currentSessionIsPersisted.set(true);
+      } else {
+        currentSessionIsPersisted.set(false);
+        id = null;
       }
     }