header.go
func (h *ResponseHeader) Peek(key string) []byte {
	k := getHeaderKeyBytes(&h.bufKV, key, h.disableNormalizing)
	return h.peek(k)
}
func (h *ResponseHeader) Peek(key string) []byte {
	k := getHeaderKeyBytes(&h.bufKV, key, h.disableNormalizing)
	return h.peek(k)
}
// Peek returns header value for the given key.
//
// Returned value is valid until the next call to ResponseHeader.
// Do not store references to returned value. Make copies instead.