Fix ObjectDisposedException
```
System.Net.Http.HttpRequestException: An error occurred while sending the request.
---> System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'System.Net.Http.StringContent'.
at System.Net.Http.HttpContent.CheckDisposed()
at System.Net.Http.HttpContent.CopyToAsync(Stream stream, TransportContext context, CancellationToken cancellationToken)
at System.Net.Http.HttpConnection.SendRequestContentAsync(HttpRequestMessage request, HttpContentWriteStream stream, CancellationToken cancellationToken)
at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
--- End of inner exception stack trace ---
at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
at System.Net.Http.HttpConnectionPool.SendWithNtConnectionAuthAsync(HttpConnection connection, HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
at System.Net.Http.DecompressionHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
at Microsoft.Extensions.Http.Logging.LoggingHttpMessageHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
at Microsoft.Extensions.Http.Logging.LoggingScopeHttpMessageHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
at System.Net.Http.HttpClient.FinishSendAsyncUnbuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)
at Emby.Dlna.PlayTo.SsdpHttpClient.SendCommandAsync(String baseUrl, DeviceService service, String command, String postData, String header, CancellationToken cancellationToken) in /home/loma/dev/jellyfin/Emby.Dlna/PlayTo/SsdpHttpClient.cs:line 41
at Emby.Dlna.PlayTo.Device.GetTransportInfo(TransportCommands avCommands, CancellationToken cancellationToken) in /home/loma/dev/jellyfin/Emby.Dlna/PlayTo/Device.cs:line 629
at Emby.Dlna.PlayTo.Device.TimerCallback(Object sender) in /home/loma/dev/jellyfin/Emby.Dlna/PlayTo/Device.cs:line 445
```
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