Improving nested_set_option helper method to support query results (checking the 'each' support)

I have a structure Brand -> Categories, I would like to limit the options in the the ```select``` element to show the categories of the current brand, like this:

```ruby
nested_set_options(@brand.categories.roots, @category) {|i, level| ....
```

with this result:
> NoMethodError: undefined method `self_and_descendants' for #<ActiveRecord::Relation:0x007ff355abbdf0>

the problem, simulating via rails console the flow in the ```nested_set_options`` helper  was this:

```ruby
1.9.3-p362 :019 >   b = Brand.find 1
1.9.3-p362 :020 > b.categories.roots.is_a? (Array)
 => false 
```
but
```ruby
1.9.3-p362 :026 > b.categories.roots.respond_to?(:each)
 => true 
```

So I changed the helper to check the input 'each' support, instead of being an array.

Let me know if I'm missing something, thanks for your work!