fix bug with product image creation

1) With Spree 2.3.2 && paperclip 4.1.1
`image = variant.images.create` 
...
and then
`image.save!`
will only create the original image, not other product image versions such as: product, large & mini

2) With ` image.attachment = open(URI)`, the product image's name is `stringio` name, that looks terrible.
`image.attachment = URI` will create the product image with original name !