Allow library to coexist with Squeel.

Scope query construction gets in a pickle if [Squeel](https://github.com/activerecord-hackery/squeel) is an included gem in a project:

```ruby
Order.in_state(:cancelled)
  #=> ...WHERE "order_transitions"."to_state" IN ("orders"."cancelled")
```

(Should be: `IN ('cancelled')`.)

Explicitly cast scope arguments to strings to avoid symbols being interpreted as references to columns on the driving table.