Fix money handling

In earlier versions of Solidus, Spree::Money doesn't expose #format, so
call it on the money object directly.

app/views/spree/products/index.rss.builder

@@ -16,7 +16,7 @@ xml.rss version: "2.0", "xmlns:g" => "http://base.google.com/ns/1.0" do
         xml.link product_url(feed_product.product)
         xml.tag! 'g:image_link', feed_product.image_link
         xml.tag! 'g:condition', feed_product.condition
-        xml.tag! 'g:price', feed_product.price.format(symbol: false, with_currency: true)
+        xml.tag! 'g:price', feed_product.price.money.format(symbol: false, with_currency: true)
       end
     end
   end