http: reuse the connection write buffer when hijacking

Previously, the connection write buffer used an extra 4kB of memory
 until the handler finished and the "conn" was garbage collected.

```
goos: linux
goarch: amd64
pkg: net/http
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
               │   old.txt   │               new.txt               │
               │   sec/op    │   sec/op     vs base                │
ServerHijack-8   13.28µ ± 2%   10.84µ ± 3%  -18.36% (p=0.000 n=10)

               │   old.txt    │               new.txt                │
               │     B/op     │     B/op      vs base                │
ServerHijack-8   15.84Ki ± 0%   11.77Ki ± 0%  -25.70% (p=0.000 n=10)

               │  old.txt   │              new.txt              │
               │ allocs/op  │ allocs/op   vs base               │
ServerHijack-8   50.00 ± 0%   48.00 ± 0%  -4.00% (p=0.000 n=10)
```

Signed-off-by: Jakob Ackermann <[email protected]>

src/net/http/server.go

@@ -322,7 +322,7 @@ func (c *conn) hijackLocked() (rwc net.Conn, buf *bufio.ReadWriter, err error) {
 	rwc = c.rwc
 	rwc.SetDeadline(time.Time{})
 
-	buf = bufio.NewReadWriter(c.bufr, bufio.NewWriter(rwc))
+	buf = bufio.NewReadWriter(c.bufr, c.bufw)
 	if c.r.hasByte {
 		if _, err := c.bufr.Peek(c.bufr.Buffered() + 1); err != nil {
 			return nil, nil, fmt.Errorf("unexpected Peek failure reading buffered byte: %v", err)