net/http: use keyed composite literal

Makes vet happy.

Updates #11041

Change-Id: I23ca413c03ff387359440af8114786cd7880a048
Reviewed-on: https://go-review.googlesource.com/27124
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/transfer.go

@@ -214,7 +214,7 @@ func (t *transferWriter) WriteBody(w io.Writer) error {
 	if t.Body != nil {
 		if chunked(t.TransferEncoding) {
 			if bw, ok := w.(*bufio.Writer); ok && !t.IsResponse {
-				w = &internal.FlushAfterChunkWriter{bw}
+				w = &internal.FlushAfterChunkWriter{Writer: bw}
 			}
 			cw := internal.NewChunkedWriter(w)
 			_, err = io.Copy(cw, t.Body)