Fix interface inconsistency SecurableDuplexPipe (cosullivan#240)

* Fix interface inconsistency SecurableDuplexPipe

* Update CustomEndpointListenerExample.cs

Author: tinohager

Examples/SampleApp/Examples/CustomEndpointListenerExample.cs

@@ -1,6 +1,7 @@
 using System;
 using System.IO;
 using System.IO.Pipelines;
+using System.Net.Security;
 using System.Security.Authentication;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
@@ -95,6 +96,9 @@ public void Dispose()
             public PipeWriter Output => _securableDuplexPipe.Output;
 
             public bool IsSecure => _securableDuplexPipe.IsSecure;
+
+            /// <inheritdoc />
+            public SslProtocols SslProtocol => (_securableDuplexPipe as SslStream)?.SslProtocol ?? SslProtocols.None;
         }
 
         public sealed class LoggingPipeReader : PipeReader

Src/SmtpServer/IO/ISecurableDuplexPipe.cs

@@ -25,5 +25,10 @@ public interface ISecurableDuplexPipe : IDuplexPipe, IDisposable
         /// Returns a value indicating whether or not the current pipeline is secure.
         /// </summary>
         bool IsSecure { get; }
+
+        /// <summary>
+        /// Returns the used SslProtocol of a secure pipeline
+        /// </summary>
+        SslProtocols SslProtocol { get; }
     }
 }

Src/SmtpServer/IO/SecurableDuplexPipe.cs

@@ -29,13 +29,7 @@ internal SecurableDuplexPipe(Stream stream, Action disposeAction)
             Output = PipeWriter.Create(_stream);
         }
 
-        /// <summary>
-        /// Upgrade to a secure pipeline.
-        /// </summary>
-        /// <param name="certificate">The X509Certificate used to authenticate the server.</param>
-        /// <param name="protocols">The value that represents the protocol used for authentication.</param>
-        /// <param name="cancellationToken">The cancellation token.</param>
-        /// <returns>A task that asynchronously performs the operation.</returns>
+        /// <inheritdoc />
         public async Task UpgradeAsync(X509Certificate certificate, SslProtocols protocols, CancellationToken cancellationToken = default)
         {
             var sslStream = new SslStream(_stream, true);
@@ -100,14 +94,10 @@ public void Dispose()
         /// </summary>
         public PipeWriter Output { get; private set; }
 
-        /// <summary>
-        /// Returns a value indicating whether or not the current pipeline is secure.
-        /// </summary>
+        /// <inheritdoc />
         public bool IsSecure => _stream is SslStream;
 
-        /// <summary>
-        /// Returns the used SslProtocol of a secure pipeline
-        /// </summary>
+        /// <inheritdoc />
         public SslProtocols SslProtocol => (_stream as SslStream)?.SslProtocol ?? SslProtocols.None;
     }
 }