html: Use existing standard library interface internally.

Now that Go 1.1 is out, commit 3651a440a7
can be reverted.

Change-Id: I7ac8478aafaa5067630e99cec9eca59792107892
Reviewed-on: https://go-review.googlesource.com/11612
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

html/render.go

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