Allow users to override handleResponse on HttpBackOffUnsuccessfulResponseHandler (#560)

Author: chingor13

google-http-client/src/main/java/com/google/api/client/http/HttpBackOffUnsuccessfulResponseHandler.java

@@ -136,7 +136,7 @@ public HttpBackOffUnsuccessfulResponseHandler setSleeper(Sleeper sleeper) {
    * {@link Sleeper#sleep(long)} will be made.
    * </p>
    */
-  public final boolean handleResponse(
+  public boolean handleResponse(
       HttpRequest request, HttpResponse response, boolean supportsRetry) throws IOException {
     if (!supportsRetry) {
       return false;