Check revoked certificates by default

Set ServicePointManager.CheckCertificateRevocationList to true in the
WebRequestHelper to enable revoked certificates by default. This is
a no-op on mono:

c5b88ec4f3/mcs/class/System/System.Net/ServicePointManager.cs (L197)

On macOS, mono will use native APIs so a revoked certificate will
have SslPolicyErrors.

Mono.Addins.Setup/Mono.Addins.Setup/WebRequestHelper.cs

@@ -37,6 +37,14 @@ namespace Mono.Addins.Setup
 	{
 		static Func<Func<HttpWebRequest>, Action<HttpWebRequest>,CancellationToken,HttpWebResponse> _handler;
 
+		static WebRequestHelper()
+		{
+			// Online certificate revocation check is not supported on Mono:
+			// https://github.com/mono/mono/blob/c5b88ec4f323f2bdb7c7d0a595ece28dae66579c/mcs/class/System/System.Net/ServicePointManager.cs#L197
+			// However on macOS Mono uses native APIs so a revoked certificate will have SslPolicyErrors.
+			ServicePointManager.CheckCertificateRevocationList = true;
+		}
+
 		/// <summary>
 		/// Sets a custom request handler that can handle requests for authenticated proxy servers.
 		/// </summary>