Force TLS1.2 support as requested by Slack

Author: gpailler

SlackAPI/SlackClientBase.cs

@@ -14,6 +14,14 @@ public abstract class SlackClientBase
         private readonly HttpClient httpClient;
         public string APIBaseLocation { get; set; } = "https://slack.com/api/";
 
+        static SlackClientBase()
+        {
+#if !NETSTANDARD1_3 && !NETSTANDARD1_6
+            // Force Tls 1.2 for Slack
+            ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
+#endif
+        }
+
         protected SlackClientBase()
         {
             this.httpClient = new HttpClient();