Fix string encoding problems from the geoip library

Strings that come out of GeoIP are sometimes UTF-8, sometimes
ISO-8859-1, sometimes falsely-labeled ASCII-8BIT. For example:

    [15] pry(#<LogStash::Event>)> (to_hash["geoip"].to_a + to_hash["pants"].to_a).collect { |k,v| p k => (v.encoding rescue v.class) }
    {"ip"=>#<Encoding:UTF-8>}
    {"country_code2"=>#<Encoding:UTF-8>}
    {"country_code3"=>#<Encoding:UTF-8>}
    {"country_name"=>#<Encoding:UTF-8>}
    {"continent_code"=>#<Encoding:UTF-8>}
    {"region_name"=>#<Encoding:ASCII-8BIT>}
    {"city_name"=>#<Encoding:ISO-8859-1>}
    {"timezone"=>#<Encoding:UTF-8>}
    {"real_region_name"=>#<Encoding:UTF-8>}
    {"number"=>#<Encoding:ASCII-8BIT>}
    {"asn"=>#<Encoding:ASCII-8BIT>}

In testing, I found that the strings with ASCII-8BIT encoding were
actually mislabeled and were ISO8859-1.

This patch converts any non-UTF-8 encoding, hopefully correctly, to
UTF-8. I also fixed a bug in the tests.

- Tests written in #1054 now pass. Hurray!
- Fixes LOGSTASH-1354, LOGSTASH-1372, and LOGSTASH-1853