ISelectClientCertificateCallback - Update xml doc, return null means no certificate selected.

Update WinForms Example to deal with user cancellation of certificate selection UI

Resolves #2853

Author: amaitland

CefSharp.WinForms.Example/Handlers/WinFormsRequestHandler.cs

@@ -43,10 +43,17 @@ protected override bool OnSelectClientCertificate(IWebBrowser chromiumWebBrowser
             control.InvokeOnUiThreadIfRequired(delegate ()
             {
                 var selectedCertificateCollection = X509Certificate2UI.SelectFromCollection(certificates, "Certificates Dialog", "Select Certificate for authentication", X509SelectionFlag.SingleSelection);
-
-                //X509Certificate2UI.SelectFromCollection returns a collection, we've used SingleSelection, so just take the first
-                //The underlying CEF implementation only accepts a single certificate
-                callback.Select(selectedCertificateCollection[0]);
+                if (selectedCertificateCollection.Count > 0)
+                {
+                    //X509Certificate2UI.SelectFromCollection returns a collection, we've used SingleSelection, so just take the first
+                    //The underlying CEF implementation only accepts a single certificate
+                    callback.Select(selectedCertificateCollection[0]);
+                }
+                else
+                {
+                    //User canceled no certificate should be selected.
+                    callback.Select(null);
+                }
             });
 
             return true;

CefSharp/Callback/ISelectClientCertificateCallback.cs

@@ -14,6 +14,7 @@ public interface ISelectClientCertificateCallback : IDisposable
     {
         /// <summary>
         /// Callback interface used to select a client certificate for authentication.
+        /// <see langword="null"/> value means that no client certificate should be used.
         /// </summary>
         /// <param name="selectedCert">selected certificate</param>
         void Select(X509Certificate2 selectedCert);