net/http: fix zombie connection leaked

When `Server.IdleTimeout` is not zero and after the first request, the client sends part of the data of a full request, and the data is no less than 4 bytes, the idle timeout which has been set will be reset to zero here and would never close the conn:
https://github.com/golang/go/blob/master/src/net/http/server.go#L1996

Author: lesismal

src/net/http/server.go

@@ -1989,9 +1989,7 @@ func (c *conn) serve(ctx context.Context) {
 
 		if d := c.server.idleTimeout(); d != 0 {
 			c.rwc.SetReadDeadline(time.Now().Add(d))
-			if _, err := c.bufr.Peek(4); err != nil {
-				return
-			}
+			return
 		}
 		c.rwc.SetReadDeadline(time.Time{})
 	}