code: 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
  }
docs: // Host returns host part, i.e. aaa.com of http://aaa.com/foo/bar?baz=123#qwe .
  //
  // Host is always lowercased.