httpclientを使ってproxy経由でhttps接続する方法について

javaでhttpclientを使い、proxy経由でアクセスしたいのですが下記エラーが解消できません。
どうすれば解消できるでしょうか。

【実行環境】
java8
org.apache.httpcomponents:httpclient v4.3
プロキシサーバーはherokuのfixieアドオンを使用

サンプルとして下記ページを参照
apacheサンプル

※fixieのサンプルも試したがうまくいきませんでした。

【実行コード】

PROXY_HOST = "velodrome.usefixie.com";//fixieのホスト名

    public void test() throws ClientProtocolException, IOException {
        CredentialsProvider credsProvider = new BasicCredentialsProvider();
        credsProvider.setCredentials(
                new AuthScope(PROXY_HOST, 80),
                new UsernamePasswordCredentials(PROXY_USER, PROXY_PASSWORD));
        CloseableHttpClient httpclient = HttpClients.custom()
                .setDefaultCredentialsProvider(credsProvider).build();
        try {
            HttpHost target = new HttpHost("www.verisign.com", 443, "https");
            HttpHost proxy = new HttpHost(PROXY_HOST, 80);

            RequestConfig config = RequestConfig.custom()
                .setProxy(proxy)
                .build();
            HttpGet httpget = new HttpGet("/");
            httpget.setConfig(config);

            System.out.println("Executing request " + httpget.getRequestLine() + " to " + target + " via " + proxy);

            CloseableHttpResponse response = httpclient.execute(target, httpget);
            try {
                System.out.println("----------------------------------------");
                System.out.println(response.getStatusLine());
                EntityUtils.consume(response.getEntity());
            } finally {
                response.close();
            }
        } finally {
            httpclient.close();
        }

【発生エラー】

2016-05-19 22:06:20.593  INFO 2350 --- [nio-8080-exec-1] o.apache.http.impl.execchain.RetryExec   : I/O exception (org.apache.http.NoHttpResponseException) caught when processing request: The target server failed to respond
2016-05-19 22:06:20.594  INFO 2350 --- [nio-8080-exec-1] o.apache.http.impl.execchain.RetryExec   : Retrying request
2016-05-19 22:06:20.963  INFO 2350 --- [nio-8080-exec-1] o.apache.http.impl.execchain.RetryExec   : I/O exception (org.apache.http.NoHttpResponseException) caught when processing request: The target server failed to respond
2016-05-19 22:06:20.963  INFO 2350 --- [nio-8080-exec-1] o.apache.http.impl.execchain.RetryExec   : Retrying request
2016-05-19 22:06:21.321  INFO 2350 --- [nio-8080-exec-1] o.apache.http.impl.execchain.RetryExec   : I/O exception (org.apache.http.NoHttpResponseException) caught when processing request: The target server failed to respond
2016-05-19 22:06:21.322  INFO 2350 --- [nio-8080-exec-1] o.apache.http.impl.execchain.RetryExec   : Retrying request
2016-05-19 22:06:21.661 ERROR 2350 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception

org.apache.http.NoHttpResponseException: The target server failed to respond
    at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:143) ~[httpclient-4.3.jar:4.3]
    at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:57) ~[httpclient-4.3.jar:4.3]
    at org.apache.http.impl.io.AbstractMessageParser.parse(AbstractMessageParser.java:261) ~[httpcore-4.4.4.jar:4.4.4]
以後省略

以上、よろしくお願いします。