[release-branch.go1.9] strings: fix encoding of \u0080 in map

Fix encoding of PAD (U+0080) which has the same value as utf8.RuneSelf
being incorrectly encoded as \x80 in strings.Map due to using <= instead
of a < comparison operator to check one byte encodings for utf8.

Fixes golang/go#25573

Change-Id: Ib6c7d1f425a7ba81e431b6d64009e713d94ea3bc
Reviewed-on: https://go-review.googlesource.com/111286


Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 8c62fc0c)
Reviewed-on: https://go-review.googlesource.com/114636


Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>