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 Looking at the schema.org documentation, it should accept either a relative or absolute URL. The examples contain both relative and absolute URL's. I confirmed this with some testing with Google's structured data tool.

On the plus side, I found that price was not specific correctly, so I moved that in to the offers section, which is correct.