response.write(chunk[, encoding][, callback])
chunk<string> | <Buffer> | <Uint8Array>encoding<string>callback<Function>- 返回：<boolean>
如果调用此方法而 response.writeHead() 尚未被调用，它将切换到隐式头模式并刷新隐式头。
【If this method is called and response.writeHead() has not been called,
it will switch to implicit header mode and flush the implicit headers.】
这会发送响应主体的一部分。此方法可以被多次调用以提供主体的连续部分。
【This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.】
在 node:http 模块中，当请求是 HEAD 请求时，响应体会被省略。类似地，204 和 304 响应_必须不_包含消息体。
【In the node:http module, the response body is omitted when the
request is a HEAD request. Similarly, the 204 and 304 responses
must not include a message body.】
chunk 可以是字符串或缓冲区。如果 chunk 是字符串，第二个参数指定如何将其编码为字节流。默认情况下，encoding 为 'utf8'。当这块数据被刷新时，callback 会被调用。
这是原始的 HTTP 正文，与可能使用的更高级的多部分正文编码无关。
【This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.】
第一次调用 response.write() 时，它会将缓冲的头信息和响应体的第一块发送给客户端。第二次调用 response.write() 时，Node.js 假设数据将以流的方式发送，并会单独发送新数据。也就是说，响应会缓冲到响应体的第一块为止。
【The first time response.write() is called, it will send the buffered
header information and the first chunk of the body to the client. The second
time response.write() is called, Node.js assumes data will be streamed,
and sends the new data separately. That is, the response is buffered up to the
first chunk of the body.】
如果所有数据都成功刷新到内核缓冲区，则返回 true。如果所有或部分数据被排队在用户内存中，则返回 false。当缓冲区再次空闲时，会触发 'drain' 事件。
【Returns true if the entire data was flushed successfully to the kernel
buffer. Returns false if all or part of the data was queued in user memory.
'drain' will be emitted when the buffer is free again.】