header.go
func (h *RequestHeader) AddBytesK(key []byte, value string) {
	h.Add(b2s(key), value)
}
func (h *RequestHeader) AddBytesK(key []byte, value string) {
	h.Add(b2s(key), value)
}
// AddBytesK adds the given 'key: value' header.
//
// Multiple headers with the same key may be added with this function.
// Use SetBytesK for setting a single header for the given key.