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)
@biyeun This should call encode, and use 'iso-8859-1' instead of 'ascii'. Also the build is failing on that unicel test - did you end up getting to the bottom of that?