#dup with unslugged model causes undefined method error

In lib/friendly_id/base.rb:252, as introduced in SHA: aa3b4a69fbe1bed107312195bf1a4345489dd770:

```
    def dup
      super.tap { |duplicate| duplicate.slug = nil }
    end
```

However, suppose we have a User with just `friendly_id :login` - no extra slug. #slug= is not defined, and therefore some_user.dup fails.

My patch just makes the dup check if slug= is defined before using it.