go.net/html: don't use Go tip io.ByteWriter

So that Go 1.0 user could also use this package.
Fixes golang/go#4931.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/7424044

html/render.go

@@ -14,7 +14,7 @@ import (
 
 type writer interface {
 	io.Writer
-	io.ByteWriter
+	WriteByte(c byte) error // in Go 1.1, use io.ByteWriter
 	WriteString(string) (int, error)
 }