PlaceOrderAsync throwing exception on Poloniex after changing to System.Net.Http.HttpClient.

[BZ-CO]

Exception is thrown after calling PlaceOrderAsync method on Poloniex.
I've also tested same method on Binance and it worked fine.
It happened after this commit. [babff91]

Exception details:
InvalidOperationException
ExchangeSharp.APIRequestMaker.InternalHttpWebRequest.AddHeader(String header, String value) in D:\ExchangeSharp-master\src\ExchangeSharp\API\Common\APIRequestMaker.cs:line 93
Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.

The problematic header seems to be content-length

Solution:
Changing Request.Headers.Add(header, value); to Request.Headers.TryAddWithoutValidation(header, value);