user's guesses are checked and marked as complete

this seems like a bit of a weird way to go about this. i feel like it's not obvious from ```guesses_controller#update``` that ```GuessScorer``` changes the state of your guess -- and i think you don't want it to. Maybe you should have a method called ```score_guess``` that returns ```true``` or ```false``` and you update in your controller like:
```ruby
guess.update(correct: score(guess, user_answer
```
i'm not totally sure, though