changing weights of products/variants doesn't bust cache

[brchristian]

I notice that the retrieve_rates_from_cache method uses a cache key that looks at the Order, the Destination, etc...

However, it does not include the Products or Variants of the Order into the MD5. So if the weight of a product or variant is changed on the backend, users might see obsolete cached rates.

I'd suggest we throw all of the included products and variants into the hash, so that if any of them are updated, we expire this shipping rate cache, just to be safe.