Define respond_to_missing? when overriding method_missing

Hi,

I hope the title is clear.

I've defined `respond_to_missing?` for `Lotus::Utils::String` and `Lotus::Utils::Hash`.

See http://robots.thoughtbot.com/always-define-respond-to-missing-when-overriding and http://blog.marc-andre.ca/2010/11/15/methodmissing-politely/
Can you please keep the style consistent with `Minitest::Spec`? Thanks.