uri.go
func (u *URI) Host() []byte {
	if len(u.host) == 0 && u.h != nil {
		u.host = append(u.host[:0], u.h.Host()...)
		lowercaseBytes(u.host)
		u.h = nil
	}
	return u.host
}
func (u *URI) Host() []byte {
	if len(u.host) == 0 && u.h != nil {
		u.host = append(u.host[:0], u.h.Host()...)
		lowercaseBytes(u.host)
		u.h = nil
	}
	return u.host
}
// Host returns host part, i.e. aaa.com of http://aaa.com/foo/bar?baz=123#qwe .
//
// Host is always lowercased.