TransferEncodingChunked may return true after being set to false

[MihaZupan]

The TransferEncodingChunked getter returns true if any of the values are "chunked".
But the setter only removes the first one.

var headers = new HttpRequestMessage().Headers;

headers.TransferEncoding.Add(new TransferCodingHeaderValue("chunked"));
headers.TransferEncoding.Add(new TransferCodingHeaderValue("chunked"));

headers.TransferEncodingChunked = false;
Console.WriteLine(headers.TransferEncodingChunked); // True

We should probably change the setter to remove all occurrences such that setting the property to false actually means it's now false.

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