$client

HttpClient，发送http请求。

例子：

    /**
     * 协程的httpclient测试
     */
    public function http_test_httpClient()
    {
        $httpClient = yield $this->client->coroutineGetHttpClient('http://localhost:8081');
        $result = yield $httpClient->coroutineGet("/TestController/test_request", ['id' => 123]);
        $this->http_output->end($result);
    }