norman/friendly_id — pull request #518 by saizai (2014-01-23 21:26:32+00:00)
Title: #dup with unslugged model causes undefined method error

In lib/friendly_id/base.rb:252:

```
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.