code: 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
  }
docs: // Host returns Host header value.