Auto logout on Garbage Collection

src/main/java/ley/untis/UntisClient.java

@@ -78,11 +78,19 @@ public class UntisClient {
     public void logout() throws APIRequestException {
         try {
             client.invoke("logout", new Object[0], Object.class);
+            auth = null;
         } catch (Throwable e) {
             throw new APIRequestException(e);
         }
     }
 
+    @Override
+    protected void finalize() throws Throwable {
+        if (auth != null) {
+            logout();
+        }
+    }
+
     /**
      * Get list of subjects
      * @return an ArrayList of subjects