public void Add(string key, IEnumerable <string> values, CustomHeaderWill behaviour)
        {
            if (!_customHeaders.TryGetValue(key, out var currentValue) || currentValue == null)
            {
                currentValue = new CustomHeader(key);
            }

            currentValue.Behaviour = behaviour;
            currentValue.Values.AddRange(values);
            _customHeaders[key] = currentValue;
        }
 public void Add(string key, string value, CustomHeaderWill behaviour)
 {
     Add(key, new [] { value }, behaviour);
 }