Add a test that ensures that downstream clients of Hashie::Mash don't break

[dblock]

We reverted a change in #217 that broke Grape (#215) and Omniauth (#216). Add a test that very explicitly ensures that doesn't happen in the future (needs a comment in the lines of "this assumption cannot change because ...).

Maybe one of @gregory @maciejmajewski @mbleigh @kenips @michaelherold can help ...