public void SetHeader()
        {
            GetRequestBuilder <int, Greeting> b = new GetRequestBuilder <int, Greeting>(baseUrl);

            b.AddHeader("header1", "badvalue1");
            b.SetHeader("header1", new List <string>()
            {
                "value1"
            });
            b.AddHeader("header2", "value2");
            b.AddHeader("header2", "value22");

            CollectionAssert.AreEqual(new List <string>()
            {
                "value1"
            }, b.GetHeader("header1"));
            CollectionAssert.AreEqual(new List <string>()
            {
                "value2", "value22"
            }, b.GetHeader("header2"));
        }