Owin.Response.AddHeader C# (CSharp) Method

        public Response AddHeader(string key, IEnumerable<string> value)
        {
            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;
        }