Quick fix for Stamps.com rate amount bug

[dustMason]

In lieu of rewriting ActiveShipping::Package#cents_from as discussed in #455, this is an effective fix for the bug described on that issue.

[kmcphillips]

I'm in to this fix.

Can you include a test to cover please?

[dustMason]

I returned to this just now to add a test and realized that there are money amounts in several places in the Stamps.com API responses that should be treated the same (as decimal values).

This, plus the 😟 feeling I get whenever a money value gets converted into a Float, is making me want to re-think this fix. I'm going to close this PR and consider a better fix for the issue.

I think this means that each Carrier subclass should be able to define its own money parsing methods. I'm not sure how else to determine that a given integer value is an amount in dollars or cents.

[kmcphillips]

Sound good. Thanks for thinking about quality.

I actually really like the idea of making each Carrier subclass override or implement its own money. cc @jonathankwok @thegedge