Compare initial state and `.in_state` args as strings, whatever they were originally

@jackfranklin could you review? This makes the `.(not_)in_state` methods less brittle to issues like #100 . It is also consistent with the code in `Statesman::Machine`, where we `.to_s` all states to avoid `:symbol != 'symbol'` problems.

@nacengineer sorry this wasn't in here before, this should avoid annoying behaviours like you spotted.