Just want to confirm that this is the test that leaks the global state. Without this, the assertion at line 163 will fail because the value of `BCrypt::Engine.cost` is changed by this test.

Setting `BCrypt::Engine.cost` to `nil` actually [resets it back to the default value](https://github.com/codahale/bcrypt-ruby/blob/f167f92c702ea6b01a9995e3089f49b95c34eecb/lib/bcrypt/engine.rb#L20-L24). So this looks like a good solution to me.