public virtual RequestWriter AddHeader(string key, IEnumerable<string> value)
{
key = key.ToLower();
if (Headers.ContainsKey(key)) {
List<string> listOfValues = new List<string>(Headers[key]);
listOfValues.AddRange(value);
SetHeader(key, listOfValues.ToArray());
} else
SetHeader(key, value);
return this;
}