Change product_url to product_path.

I'm not sure why product_url was introduced, but I'm currently encountering
issues in production where it's not respecting subdomains, HTTPS, and ports
correctly.

Nearly everything else in Spree links to stuff using the _path helpers, which
I like a lot better.
The link with the itemprop=url below this is what needs the full url for the schema.org product.  So I think the semantic markup needs a full url, but should be fine to change the actual link to be a path so long as the rich snippet still finds a url.