Formalize #amount_changed? method for deciding whether a transaction should be re-reported

[benjaminwil]

As a TaxJar account administrator, I want my transactions to be re-reported when their order amounts change so that my reporting is accurate.

More information

There are a few different ways that an amount can change, and we need to decide whether any of them should be handled in a special way:

• The order subtotal and the additional tax total both change, and the total changes as a result.
• The order subtotal and the additional tax total both change, but the total does not change.
• The order subtotal does not change but the additional tax total does change.
• The order subtotal changes but the additional tax total does not change.