Require `iconv` Before Monkeypatching

I tried `rake gem` after patched #10 and some error like `superclass mismatch for class Iconv`. Turns out that lib/sup/util.rb is declaring `Iconv` as a subclass of `Object`, while the original `Iconv` class from Ruby stdlib is a subclass of `Data` living somewhere I don't know.

This patch solves this problem for now. `Iconv` is deprecated but for now I'd really like to get a usable release. Hope this got merged.