Fix networking Mono Issue

Workaround for what is most likely mono/mono#8775 which seems as if it will never be fixed.

fCraft/Network/Heartbeat.cs

@@ -144,6 +144,7 @@ static void Beat([NotNull] SchedulerTask scheduledTask)
                 ParseHeartbeatResponse(replyString);
             } catch (Exception ex) {
                 LogHeartbeatError(state, ex);
+                heartBeatRequest.ServicePoint.CloseConnectionGroup(null);
             }
         }