转stackoverflow一段话：https://stackoverflow.com/questions/18598780/is-resp-body-close-necessary-if-we-dont-read-anything-from-the-body
When you call http.Get, the function returns a response as soon as all the HTTP headers have been read. The body of the response has not been read yet. The Response.Body is a wrapper around the network connection to the server. When you read from it, it downloads the body of the response.
.Close() tells the system that you're done with the network connection. If you have not read the response body, the default http transport closes the connection. (The transport can only re-use the connection if the body has been read, because if it reused a connection with an unread body the next request made using that connection would receive the previous request's response!)
So reading the Body is often more efficient than simply Close()ing if you're making more than one request - especially with TLS connections which are relatively expensive to create.
If you don't need the body of the response, you should use Head instead of Get. Head doesn't require reading or closing the response body.
body.Close()和把整个body读取完（读取到eof）底层会发送回收连接的通知，这个回收并不是关闭连接，可以回收到资源池以复用；
详细的代码原理可以见：https://www.jianshu.com/p/407fada3cc9d
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