Excessive rates returned from USPS compared to actual USPS site when multiple quantities of the same item is shipped.

1. Create a product shipping weight as 0.8 lbs (12.8 oz)
2. Activate USPS First-Class Mail Parcel, USPS Priority Mail 1-day, USPS Priority Mail Express
3. Order qty 4 of item in step 1.

All the rates were obtained using max_package_weight of USPS First-Class Mail Parcel which has a limit of 13 oz.   This causes the other rates to be excessive amounts.

Corrected with the following changes:
Enhanced cache key to include the max_weight allowed to ship to a country.
Modified validation/check methods to properly handle a nil value.
Modified compute_package method to invoke rate_result_key method so no longer need to duplicate compute_package method usps/base.rb