Fix hanging issue under UWP.

Source/MQTTnet/Implementations/MqttTcpChannel.Uwp.cs

@@ -64,7 +64,8 @@ namespace MQTTnet.Implementations
 
             if (_options.TlsOptions?.UseTls != true)
             {
-                await _socket.ConnectAsync(new HostName(_options.Server), _options.GetPort().ToString());
+                var asyncAction = _socket.ConnectAsync(new HostName(_options.Server), _options.GetPort().ToString());
+                await asyncAction;
             }
             else
             {
@@ -85,7 +86,8 @@ namespace MQTTnet.Implementations
                     socketProtectionLevel = SocketProtectionLevel.Tls10;
                 }
 
-                await _socket.ConnectAsync(new HostName(_options.Server), _options.GetPort().ToString(), socketProtectionLevel);
+                var asyncAction = _socket.ConnectAsync(new HostName(_options.Server), _options.GetPort().ToString(), socketProtectionLevel);
+                await asyncAction;
             }
 
             Endpoint = _socket.Information.RemoteAddress + ":" + _socket.Information.RemotePort;