String and Array types on ActiveModel conditionals

This PR updates the `#run_conditional` method so that String and Array types are supported, as they are in ActiveModel. For example, you can do:

```ruby
validates_presence_of :foobar, :if => 
    [:should_validate_things?, '@things_to_validate.include?("foobar)']
```

I realize this project is no longer being maintained, so I thought I'd just leave this here in case anyone finds it useful.