Fix regression in DlnaHttpClient
```
[18:53:50] [ERR] [25] Emby.Dlna.Main.DlnaEntryPoint: Error updating device info for 192.168.1.21 - Sonos Connect:Amp Berging
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, Boolean async, CancellationToken cancellationToken)
at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
--- End of inner exception stack trace ---
at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
at System.Net.Http.DecompressionHandler.SendAsync(HttpRequestMessage request, Boolean async, 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.<SendAsync>g__Core|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)
at Emby.Dlna.PlayTo.DlnaHttpClient.SendRequestAsync(HttpRequestMessage request, CancellationToken cancellationToken) in /home/loma/dev/jellyfin/Emby.Dlna/PlayTo/DlnaHttpClient.cs:line 47
at Emby.Dlna.PlayTo.Device.GetTransportInfo(TransportCommands avCommands, CancellationToken cancellationToken) in /home/loma/dev/jellyfin/Emby.Dlna/PlayTo/Device.cs:line 705
at Emby.Dlna.PlayTo.Device.TimerCallback(Object sender) in /home/loma/dev/jellyfin/Emby.Dlna/PlayTo/Device.cs:line 521
```
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