@Override
    public void failed(final Exception cause) {
    }
Triggered to signal the beginning of data processing.

@param response the response message head
@param contentType the content type of the response body,
                   or {@code null} if the response does not enclose a response entity.
@throws HttpException If a protocol error occurs.
@throws IOException   If an I/O error occurs.
httpclient5/src/main/java/org/apache/hc/client5/http/async/methods/AbstractCharPushConsumer.java