all: avoid bytes.NewBuffer(nil)

all: avoid bytes.NewBuffer(nil)
The practice encourages people to think this is the way to
create a bytes.Buffer when new(bytes.Buffer) or
just var buf bytes.Buffer work fine.
(html/token.go was missing the point altogether.)

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5637043