Can't add Certificate into TcpConn

[Sunyao-work]

Certificate collection is empty in code as below

try { sslStream.AuthenticateAsClient(_hostname, new X509Certificate2Collection(), enabledSslProtocols, tlsConfig.CheckCertificateRevocation); } catch (Exception ex) { throw new Exception(string.Format("{0} - {1}", ex.Message, errorMessage), ex); }

[Misiu]

I have this working.
I'm quite new to this library, but I can create PR.
@judwhite any chance you could review it?

[Misiu]

#101
@Sunyao-work I've created PR that adds support for client cert
@judwhite can you review this?