Correct failing specs

Having forked this project I did the following:

```bash
bundle install
bundle exec rspec
```

In which case I had multiple failures due to "undefined method `true?`". For some reason we are missing that matcher on rspec. I updated the specs to check for an exact match on `true` which then raised different failures due to the way constraints were returning values that were *truthy*. Because of this I then updated the constraints to always return a true or false and not a value like "example" or 0.

@lyokato Would love to know you're thoughts