Don't compare TaxJar-provided transaction floats with BigDecimal order amounts

[benjaminwil]

In our extension's reporting subscribers, we use an #amount_changed? method to check whether we need to re-report a transaction or not. When we get a transaction from TaxJar, they give us the amounts as floats. But Solidus does not use floats internally. If we can safely convert the TaxJar amounts to BigDecimal numbers, that would be ideal.