Makes taxonomies API more extensible

Makes small changes to the taxonomies API controller so it is easier to extend it via a decorator.

For example, consider the case where someone wants to restrict the taxonomies with a scope. Before this change, the only way to do it was to overwrite the index action and duplicate the internal spree code (note the call to `my_scope` in the end of the chain):

```ruby
Spree::Api::TaxonomiesController.class_eval do
  def index
    @taxonomies = Taxonomy.accessible_by(current_ability, :read).order('name').includes(:root => :children).
                  ransack(params[:q]).result.
                  page(params[:page]).per(params[:per_page]).my_scope
    respond_with(@taxonomies)
  end
end
```

But after the change, we can just append the scope and avoid duplication:

```ruby
Spree::Api::TaxonomiesController.class_eval do
  private

  alias_method :original_taxonomies, :taxonomies
  def taxonomies
    original_taxonomies.my_scope
  end
end
```

A similar argument holds for the show action.

Since no new functionality was introduced, I didn't added tests.