Fixed some asserts by using more appropriated `TestCase` methods.

Hi,

I've just noticed that you were using bare `assert`s to test the presence of several strings in the responses data. That can be improved by using the `assertIn` and `assertNotIn` methods of [`TestCase`](http://docs.python.org/2/library/unittest.html#unittest.TestCase.assertIn). Here you have a pull request with it, for your consideration.

Thanks for the nice tutorial!