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
```

Emby.Dlna/PlayTo/SsdpHttpClient.cs

@@ -101,7 +101,7 @@ namespace Emby.Dlna.PlayTo
                 LoadOptions.PreserveWhitespace);
         }
 
-        private Task<HttpResponseMessage> PostSoapDataAsync(
+        private async Task<HttpResponseMessage> PostSoapDataAsync(
             string url,
             string soapAction,
             string postData,
@@ -126,7 +126,7 @@ namespace Emby.Dlna.PlayTo
 
             options.Content = new StringContent(postData, Encoding.UTF8, MediaTypeNames.Text.Xml);
 
-            return _httpClientFactory.CreateClient(NamedClient.Default).SendAsync(options, HttpCompletionOption.ResponseHeadersRead, cancellationToken);
+            return await _httpClientFactory.CreateClient(NamedClient.Default).SendAsync(options, HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false);
         }
     }
 }