agent.createConnection(options[, callback])

• options <Object> 包含连接详细信息的选项。检查 net.createConnection() 以了解选项的格式
• callback <Function> 接收已创建套接字的回调函数
• 返回：<stream.Duplex>

生成用于 HTTP 请求的套接字/流。

🌐 Produces a socket/stream to be used for HTTP requests.

默认情况下，该函数与 net.createConnection() 相同。但是，如果需要更大的灵活性，自定义代理可以覆盖此方法。

🌐 By default, this function is the same as net.createConnection(). However, custom agents may override this method in case greater flexibility is desired.

可以通过两种方式提供套接字/流：一种是从此函数返回套接字/流，另一种是将套接字/流传递给 callback。

🌐 A socket/stream can be supplied in one of two ways: by returning the socket/stream from this function, or by passing the socket/stream to callback.

此方法保证返回一个 <net.Socket> 类的实例，该类是 <stream.Duplex> 的子类，除非用户指定了除 <net.Socket> 之外的套接字类型。

🌐 This method is guaranteed to return an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

callback 的签名是 (err, stream)。