Make SslConnection check IsEncrypted and IsSigned after authentication

For reference:

"Before sending or receiving information, the client and server should check the security services and levels provided by the SslStream to determine whether the protocol, algorithms, and strengths selected meet their requirements for integrity and confidentiality. If the current settings are not sufficient, the stream should be closed. You can check the security services provided by the SslStream using the IsEncrypted and IsSigned properties. The following table shows the elements that report the cryptographic settings used for authentication, encryption and data signing."

https://msdn.microsoft.com/en-us/library/system.net.security.sslstream(v=vs.110).aspx