Socket.Disconnect throws a PlatformNotSupportedException on coreclr/linux

#266

Author: halter73

test/Microsoft.AspNet.Server.KestrelTests/EngineTests.cs

@@ -395,7 +395,6 @@ public async Task DisconnectingClient(ServiceContext testContext)
                 var socket = new Socket(SocketType.Stream, ProtocolType.IP);
                 socket.Connect(IPAddress.Loopback, 54321);
                 await Task.Delay(200);
-                socket.Disconnect(false);
                 socket.Dispose();
 
                 await Task.Delay(200);