Sorry for the poor naming. 
After implementation `sinon.fakeServer.create`, small delay before receiving response appeared and  I have to use `waitsFor` for that reason. `assertResponse` just hides `waitsFor` mechanism under the hood. `assertRendering` is a function that contains expectation and  that will be called when response is received.

`assertResponse` renamed to `waitsForResponse`
`assertRendering` renamed to `assertToHaveBeenRendered`