SendPostRequestAsync()'s retry logic causes ObjectDisposedException

[nshenoy]

In .NET Framework, the HttpClient automatically disposes the HttpContent after the request is made (see dotnet/runtime#14612). In the RequestManager's SendPostRequestAsync() implementation, calling the RunWithRetries() method will call subsequent retry calls to the underlying client.PostAsync() to fail as the original HttpContent gets disposed after the initial failed call. This was found because our code is exactly hitting this issue with the following message: Cannot access a disposed object. Object name: 'System.Net.Http.StringContent'. Consider restructuring this code in such a way that the HttpContent is recreated on each retry.