Python Scrapy、HTTP状态代码200和重定向

我尝试使用scrapy跟踪重定向，但我遇到以下问题：

如果我没有为t.co url缩短器设置用户代理，我将获得301状态代码和此响应正文：

# 'scrapy.contrib.downloadermiddleware.useragent.UserAgentMiddleware': None,
<noscript>
    <META http-equiv="refresh" content="0;URL=http://sn.im/29m4uw5">
</noscript>
<title>http://sn.im/29m4uw5</title>
<script>
    window.opener = null; location.replace("http:\/\/sn.im\/29m4uw5")
</script>

DOWNLOADER_MIDDLEWARES = {
'scrapy.contrib.downloadermiddleware.useragent.UserAgentMiddleware': 400,
'scrapy.contrib.downloadermiddleware.redirect.MetaRefreshMiddleware': 550,
'scrapy.contrib.downloadermiddleware.redirect.RedirectMiddleware': 600}

REDIRECT_ENABLED = True
METAREFRESH_ENABLED = True

USER_AGENT = 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.95 Safari/537.36'