Gem gem update --system -V 时，服务器返回 302 和 304,是不是服务器绕了弯路？
或者 TCP 被劫持了？还是其他什么原因？
gem update --system -V
GET http://ruby.taobao.org/specs.4.8.gz
302 Moved Temporarily
GET http://rubygems-china.oss.aliyuncs.com/specs.4.8.gz
304 Not Modified
Updating rubygems-update
HEAD http://ruby.taobao.org/api/v1/dependencies
404 Not Found
GET http://ruby.taobao.org/prerelease_specs.4.8.gz
302 Moved Temporarily
GET http://rubygems-china.oss.aliyuncs.com/prerelease_specs.4.8.gz
200 OK
GET http://ruby.taobao.org/specs.4.8.gz
302 Moved Temporarily
GET http://rubygems-china.oss.aliyuncs.com/specs.4.8.gz
304 Not Modified
