In Rails 5 assigning a value which is not explicitly `true` or `false` to a boolean column transforms it to `true`.
In Rails 4 it transforms the value to `false` with the deprecation warning:

```
DEPRECATION WARNING: You attempted to assign a value which is not explicitly `true` or `false` ("nil") to a boolean column. Currently this value casts to `false`. This will change to match Ruby's semantics, and will cast to `true` in Rails 5. If you would like to maintain the current behavior, you should explicitly handle the values you would like cast to `false`.
```

This commit fixes the spec that expects to receive Rails 4 behavior.