#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.

lib/friendly_id/base.rb

@@ -249,7 +249,7 @@ def to_param
 
     # Clears slug on duplicate records when calling `dup`.
     def dup
-      super.tap { |duplicate| duplicate.slug = nil }
+      super.tap { |duplicate| duplicate.slug = nil if duplicate.respond_to?('slug=') }
     end
   end
 end