http: include DEL in the test for unprintable chars

R=adg, rsc
CC=golang-dev
https://golang.org/cl/3631041

src/pkg/http/fs.go

@@ -27,7 +27,7 @@ func isText(b []byte) bool {
 			// decoding error
 			return false
 		}
-		if 0x80 <= rune && rune <= 0x9F {
+		if 0x7F <= rune && rune <= 0x9F {
 			return false
 		}
 		if rune < ' ' {