Fix for Array#to_s behavior in Ruby 1.9.

Ruby 1.8:

```ruby
[1,2,3].to_s
 => "123"
```

Ruby 1.9:

```ruby
[1,2,3].to_s
 => "[1, 2, 3]"
```

This causes navigation_helper to render the items array incorrectly. The issue is fixed by calling to_s on each element and then joining them. It should still behave correctly in Ruby 1.8.