invoice: fix bug in DefaultInvoiceGenerator

Rate being a BigDecimal, we should really use compareTo.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
beatrix: add missing expected NextEvent.INVOICE_ADJUSTMENT events

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
beatrix: fix date in TestIntegrationInvoiceWithRepairLogic

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>