html: Use existing standard library interface internally.

Now that Go 1.1 is out, commit 3651a440a70063fcb7028c5f214b0ffb20b3770d
can be reverted.

Change-Id: I7ac8478aafaa5067630e99cec9eca59792107892
Reviewed-on: https://go-review.googlesource.com/11612
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/html/render.go b/html/render.go
index 4a833b4..d34564f 100644
--- a/html/render.go
+++ b/html/render.go
@@ -14,7 +14,7 @@
 
 type writer interface {
 	io.Writer
-	WriteByte(c byte) error // in Go 1.1, use io.ByteWriter
+	io.ByteWriter
 	WriteString(string) (int, error)
 }