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.
@JDutil thanks for pointing that out. I will investigate this further and add another commit to this pull request to address this issue.