The "cost" value was flawed for a couple of reasons.
1. Lots of messages are free, so in those instances the "cost"
doesn't tell you anything
2. The query to get the rate was expensive and we don't have
an obvious way to get it back very efficiently for large numbers
of notifications.
So we scrapped it.
Our previous test ws returning a notification without a `sent_by`
attribute, which meant that cost was always 0.
Unfortunately, this meant that returning a real value for cost was
untested and (whaddya know) it broke immediately.
Old test scenario:
- billable_units=1, sent_by=None, cost=0
New scenarios
- billable_units=0, sent_by='mmg', cost=0
- billable_units=1, sent_by='mmg', cost=1
The new 'v2' API wants to return less data than the previous one,
which was sending back tons of fields the clients never used.
This new route returns only useful information, with the JSON
response dict being built up in the model's `.serialize()` method.
Note that writing the test for this was a bit painful because of
having to treat loads of keys differently. Hopefully we think this
is a good way to write this test, because if we don't, we should
start thinking of a better way to check the values are what we
expect.
