Fix Spree::TaxonsHelper#taxon_preview not to show duplicate products

If a product belongs to nested taxons(e.g, category->sub category->sub-sub category), there's a possibility #taxon_preview returns duplicate products.

This error is caused by wrong way of use uniq method.
Here's an example to explain rails behavior.
```ruby
products = Spree::Product.limit 1
products.class
# ActiveRecord::Relation::ActiveRecord_Relation_Spree_Product
products += products
products.class
# Array
```
When activerecord's collection is merged by "+=", it returns an Array object.
So "products.uniq" doesn't work out as expected.