forcing UTF-8 encoding when returning results from MaxMind’s local db

I've had some problems using MaxMind's local database, which seems to be returning "ISO-8859-1" encoded strings. I saw you have implemented a re-encodig solution for other IP lookup options, but `maxmind_local.rb` was still having this issue.

I've added a simple `inject` enumerator which makes sure all `Strings` are encoded in UTF-8 before returning.
I no longer have problems looking up IPs from Montréal.