net: fix case insensitivity lookup for local database such as /etc/hosts

net: fix case insensitivity lookup for local database such as /etc/hosts

The previous change for #12806 modified internal lookup tables and made
LookupAddr return forcibly lowercased host names by accident.

This change fixes the issue again without any behavioral change for
LookupAddr and adds missing test cases for lookupStaticHost and
lookupStaticAddr.

Updates #12806.
Fixes #13359.

Change-Id: Ifff4741cd79eb8b320b1b0f8c5e02b3a167c9fa8
Reviewed-on: https://go-review.googlesource.com/17217
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>