code: func ReqHostIs(hosts ...string) ReqConditionFunc {
  	hostSet := make(map[string]bool)
  	for _, h := range hosts {
  		hostSet[h] = true
  	}
  	return func(req *http.Request, ctx *ProxyCtx) bool {
  		_, ok := hostSet[req.URL.Host]
  		return ok
  	}
  }
docs: // ReqHostIs returns a ReqCondition, testing whether the host to which the request is directed to equal
  // to one of the given strings