header.go
func (h *RequestHeader) Host() []byte {
	if len(h.host) > 0 {
		return h.host
	}
	if !h.rawHeadersParsed {
		// fast path without employing full headers parsing.
		host := peekRawHeader(h.rawHeaders, strHost)
		if len(host) > 0 {
			h.host = append(h.host[:0], host...)
			return h.host
		}
	}

	// slow path.
	h.parseRawHeaders()
	return h.host
}
func (h *RequestHeader) Host() []byte {
	if len(h.host) > 0 {
		return h.host
	}
	if !h.rawHeadersParsed {
		// fast path without employing full headers parsing.
		host := peekRawHeader(h.rawHeaders, strHost)
		if len(host) > 0 {
			h.host = append(h.host[:0], host...)
			return h.host
		}
	}

	// slow path.
	h.parseRawHeaders()
	return h.host
}
// Host returns Host header value.