http.ClientRequest 类
此对象从 http.request() 内部创建并返回。
它表示正在进行的请求，其标头已入队。
使用 setHeader(name, value)、getHeader(name)、removeHeader(name) API 时，标头仍然是可变的。
实际标头将与第一个数据块一起发送或在调用 request.end() 时发送。
要获得响应，则将 'response' 的监听器添加到请求对象。
当接收到响应头时，则请求对象会触发 'response'。
'response' 事件使用一个参数执行，该参数是 http.IncomingMessage 的实例。
在 'response' 事件期间，可以向响应对象添加监听器；特别是监听 'data' 事件。
如果没有添加 'response' 句柄，则响应将被完全丢弃。
但是，如果添加了 'response' 事件句柄，则必须消费响应对象中的数据，方法是在有 'readable' 事件时调用 response.read()，或者添加 'data' 句柄，或者调用 .resume() 方法。
在数据被消费之前，不会触发 'end' 事件。
此外，在读取数据之前，其会消耗内存，最终可能导致进程内存不足的错误。
为了向后兼容，如果注册了 'error' 监听器，则 res 只会触发 'error'。
Node.js 不会检查内容长度和已经传输的正文的长度是否相等。
- Extends: <http.OutgoingMessage>
This object is created internally and returned from http.request(). It
represents an in-progress request whose header has already been queued. The
header is still mutable using the setHeader(name, value),
getHeader(name), removeHeader(name) API. The actual header will
be sent along with the first data chunk or when calling request.end().
To get the response, add a listener for 'response' to the request object.
'response' will be emitted from the request object when the response
headers have been received. The 'response' event is executed with one
argument which is an instance of http.IncomingMessage.
During the 'response' event, one can add listeners to the
response object; particularly to listen for the 'data' event.
If no 'response' handler is added, then the response will be
entirely discarded. However, if a 'response' event handler is added,
then the data from the response object must be consumed, either by
calling response.read() whenever there is a 'readable' event, or
by adding a 'data' handler, or by calling the .resume() method.
Until the data is consumed, the 'end' event will not fire. Also, until
the data is read it will consume memory that can eventually lead to a
'process out of memory' error.
For backward compatibility, res will only emit 'error' if there is an
'error' listener registered.
Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not.