Update sinon expectation to work with sinon version >= 1.8

If you update the sinon dependency to 1.8 or newer, the tests will fail with `TypeError: argument at index 3 is not a function: undefined`. This is because the behaviour of `callsArgWith` has changed, and it's likely that `callsArgWith` wasn't the right thing to be calling here anyway.

I've changed the expectations to do what I think they should be doing, and which continues to work with newer versions of sinon.