Expand test coverage, switch to ensure()
Expand test coverage, switch to ensure()

I started writing these tests for my own implementation of streams, using the `ensure` assertion library. I love the literate style of the assertions. I've merged my tests in with yours, switching to `ensure` in every case except `self.assertAlmostEquals()`.

With our tests combined, we reach 99% coverage of `streams.stream`!

9seconds/streams,9seconds/streams