protected void customizeConnection(URLConnection con) throws IOException {
        ResourceUtils.useCachesIfNecessary(con);
        if (con instanceof HttpURLConnection) {
            customizeConnection((HttpURLConnection) con);
        }
    }
Customize the given {@link URLConnection}, obtained in the course of an
{@link #exists()}, {@link #contentLength()} or {@link #lastModified()} call.
<p>Calls {@link ResourceUtils#useCachesIfNecessary(URLConnection)} and
delegates to {@link #customizeConnection(HttpURLConnection)} if possible.
Can be overridden in subclasses.

@param con the URLConnection to customize
@throws IOException if thrown from URLConnection methods
src/main/java/spark/resource/AbstractFileResolvingResource.java