header.go
func (h *ResponseHeader) Set(key, value string) {
	initHeaderKV(&h.bufKV, key, value, h.disableNormalizing)
	h.SetCanonical(h.bufKV.key, h.bufKV.value)
}
func (h *ResponseHeader) Set(key, value string) {
	initHeaderKV(&h.bufKV, key, value, h.disableNormalizing)
	h.SetCanonical(h.bufKV.key, h.bufKV.value)
}
// Set sets the given 'key: value' header.
//
// Use Add for setting multiple header values under the same key.