Enable persistent connection for HTTP file upload

This seems to fix the following error issued when trying to upload files > ~200Kb to certain HTTP servers

>Message:
>Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
>
>StackTrace:
   >at System.Net.Sockets.NetworkStream.Write(Byte[] buffer, Int32 offset, Int32 size)
   >at System.Net.PooledStream.Write(Byte[] buffer, Int32 offset, Int32 size)
   >at System.Net.ConnectStream.InternalWrite(Boolean async, Byte[] buffer, Int32 offset, Int32 size, AsyncCallback callback, Object state)
   >at System.Net.ConnectStream.Write(Byte[] buffer, Int32 offset, Int32 size)
   >at UploadersLib.Uploader.TransferData(Stream dataStream, Stream requestStream)
   >at UploadersLib.Uploader.UploadData(Stream dataStream, String url, String fileName, String fileFormName, Dictionary`2 arguments, NameValueCollection headers, CookieCollection cookies, ResponseType responseType, HttpMethod method)