Add SslVerification to HttpClient (#400)

***NO_CI***

Author: josesimoes

nanoFramework.System.Net.Http/Http/HttpClient.cs

@@ -130,6 +130,14 @@ public TimeSpan Timeout
         /// </remarks>
         public SslProtocols SslProtocols { get; set; } = SslProtocols.Tls12;
 
+        /// <summary>
+        /// Gets or sets the TLS/SSL verification mode used by the <see cref="HttpClient"/> class.
+        /// </summary>
+        /// <remarks>
+        /// Default value is <see cref="SslVerification.CertificateRequired"/>.
+        /// </remarks>
+        public SslVerification SslVerification { get; set; } = SslVerification.CertificateRequired;
+
         #region Constructors
 
         /// <summary>
@@ -408,6 +416,7 @@ private HttpResponseMessage SendWorker(HttpRequestMessage request, HttpCompletio
                 clientHandler.SetWebRequestTimeout(_timeout);
                 clientHandler.SetWebRequestSslProcol(SslProtocols);
                 clientHandler.SetWebRequestHttpAuthCert(HttpsAuthentCert);
+                clientHandler.SetWebRequestSslVerification(SslVerification);
             }
 
             HttpResponseMessage response = base.Send(request);

nanoFramework.System.Net.Http/Http/HttpClientHandler.cs

@@ -26,6 +26,7 @@ public partial class HttpClientHandler : HttpMessageHandler
         private X509Certificate _caCert;
         private X509Certificate _clientCert;
         private ClientCertificateOption _clientCertificateOptions = ClientCertificateOption.Manual;
+        private SslVerification _sslVerification;
 
         /// <summary>
         /// Gets or sets a value that indicates if the certificate is automatically picked from the certificate store or if the caller is allowed to pass in a specific client certificate.
@@ -306,6 +307,7 @@ private HttpWebRequest CreateWebRequest(HttpRequestMessage request)
 
             wr.SslProtocols = _sslProtocols;
             wr.HttpsAuthentCert = _caCert;
+            wr.SslVerification = _sslVerification;
 
             if (ClientCertificateOptions == ClientCertificateOption.Manual)
             {
@@ -392,5 +394,10 @@ internal void SetWebRequestHttpAuthCert(X509Certificate certificate)
         {
             _caCert = certificate;
         }
+
+        internal void SetWebRequestSslVerification(SslVerification sslVerification)
+        {
+            _sslVerification = sslVerification;
+        }
     }
 }

nanoFramework.System.Net.Http/Http/System.Net.HttpWebRequest.cs

@@ -270,6 +270,11 @@ protected override void Dispose(bool disposing)
             '\t',
             '\n'};
 
+        /// <summary>
+        /// Exposes this property from <see cref="SslStream.SslVerification"/>.
+        /// </summary>
+        private SslVerification _sslVerification;
+
         /// <summary>
         /// The maximum length, in kilobytes (1024 bytes), of the response
         /// headers.
@@ -378,6 +383,14 @@ public X509Certificate HttpsAuthentCert
             set { m_caCert = value; }
         }
 
+        /// <summary>
+        /// Gets or sets 
+        /// </summary>
+        public SslVerification SslVerification
+        {
+            get { return _sslVerification; }
+            set { _sslVerification = value; }
+        }
 
         /// <summary>
         /// Gets or sets the TLS/SSL protocol used by the <see cref="HttpWebRequest"/> class.
@@ -1497,6 +1510,8 @@ private InputNetworkStreamWrapper EstablishConnection(Uri proxyServer, Uri targe
                     // Once connection established need to create secure stream and authenticate server.
                     SslStream sslStream = new SslStream(retStream.m_Socket);
 
+                    sslStream.SslVerification = _sslVerification;
+
                     // Throws exception if it fails
                     sslStream.AuthenticateAsClient(m_originalUrl.Host, null, m_caCert, m_sslProtocols);