sitemap index remains .xml when each file is compressed with .gz extension

[mandye]

Shoud add one option to

def add(map)
raise ArgumentError, 'XmlSitemap::Map object requred!' unless map.kind_of?(XmlSitemap::Map)
raise ArgumentError, 'Map is empty!' if map.empty?

  @maps << {
    :loc     => map.index_url(@offsets[map.group]),
    :lastmod => map.created_at.utc.iso8601
  }
  @offsets[map.group] += 1

end

add '.gz' to index entries, if people choose :gzip => true for each map

[sosedoff]

Can you work on this patch?