Fixed issue with duplicate thumbnails on ProductController#show

[iloveitaly]

Spree 1.1.4 changes the way thumbnails were displayed. I didn't look into what exactly changed, but I think Product#images and Product#variant_images both returned the master variant image. This makes sense, but the thumbnail's partial was not updated to reflect this – in my case I was getting duplicate thumbnail display. This PR fixed the duplicate thumbnail display issue and cleans up the thumbnails partial.

Also, there were references to both @product and product, I switched to using the instance variable for consistency's sake.

This applies to master, 1-2-stable, and 1-1-stable.

[radar]

I will only be applying this patch to master, 1-3-stable and 1-2-stable, as those are the only versions of Spree that we are maintaining now. Unless there's a security bug in 1-1-stable, we won't be patching it any further.