public static HttpRequestMessage AcceptCharset(this HttpRequestMessage req, Encoding charset) => req.AcceptCharset(charset.WebName);
public static HttpRequestMessage AcceptUtf8(this HttpRequestMessage req) => req.AcceptCharset(System.Text.Encoding.UTF8);
public static HttpRequestMessage AcceptCharset(this HttpRequestMessage req, string charset) => req.AcceptCharset(new StringWithQualityHeaderValue(charset));