Refactor the tests for charge_on_balanced

Picking up from #2059, this commit refactors the existing tests for
charge_on_balanced (which is where the bug in #2057 lives) to use VCR
instead of mock objects. I was hoping it would make the tests cleaner
but it looks like I don't quite fully understand how to use VCR yet. :-/
Merge pull request #2063 from gittip/refactor-charge_on_balanced-tests

Refactor charge_on_balanced tests