Fix API url when editing image alt text in admin

[loicginoux]

When you upload a product image in BO, you can update the alternative text and save the change.
When doing this the url generated is
/en/api/variants/<PRODUCT_SLUG>/images/4

This is not actually causing any bug on the default solidus behaviour because the variant is not used in
Spree::Api::ImagesController#update but if we would like to use the variant via the method Spree::Api::ImagesController#scope for example, it would produce an error (this is the case in our app for example)

     def scope
        if params[:product_id]
          Spree::Product.friendly.find(params[:product_id])
        elsif params[:variant_id]
          Spree::Variant.find(params[:variant_id])
        end
      end

Example of log when saving the image alternative text:

Started PUT "/en/api/variants/75-aoc-esp-rioja-crianza-rg-marques-caceres-13c6-acq/images/19" for 83.60.66.210 at 2018-03-12 11:38:56 +0100
Processing by Spree::Api::ImagesController#update as JSON
   Parameters: {"image"=>{"alt"=>"seigneur dagil"}, "variant_id"=>"75-aoc-esp-rioja-crianza-rg-marques-caceres-13c6-acq", "id"=>"19", "locale"=>"en"}

As you see from the log we could not find the variant via the product slug.

Replacing the path by
/en/api/products/<PRODUCT_SLUG>/images/4
should fix this issue.

[jhawthorn]

This change looks correct. Also, maybe we should be using that scope in #update, the route we're hitting shouldn't work here (let's merge this and fix that in a separate PR)

[kennyadsl]

Thanks!