Billing domain filter fix

Fixes a **PRIORITY 2** Billing Bug: 
http://manage.dimagi.com/default.asp?132430

adds:
- regression test so this gets caught in the future
- fix to repair old `SMSBillables` with unspecified domains.
- performance improvement to `DomainFilter` in the billables report (yeah, this could have been a separate PR :P)
(offlined with @biyeun - this is because we can't link it to a domain so we don't know who to bill)