我使用了私有文件服务器是https但是私有证书，请问如何配置

Issue

1. What problem do you get?无法下载https服务器的文件，文件服务器是私有的，证书也是私有的
2. Which version of FileDownloader are you using when you produce such problem?1.7.5
3. How to reproduce such problem?
4. Do you set FileDownloadLog.NEED_LOG=true?
5. Could you please reproduce this problem and provide all main process and :filedownloader process logcat
6. Can you fix it by yourself and request PR, if not, what's problem do you get when you try to fix it

P.S. If you don't know how to get :filedownloader process, it's recommended to using pidcat to just filter all your application logcat, or define process.non-separate=true on filedownloader.properties

请在Issue前认真的跟进上面提到的建议，这样将可以极大的加快你遇到问题的处理。

屏幕下面会弹出 error 1 javax.net.ssl.SSL.HandshakeException:java.security.cert.CertPathValidatorException