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 #25242

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>