Add internal document asserts to integration tests

While there are unit tests to make sure the internal document is updated correctly after an atomic update, they all use Mock. So long as my assumptions about the behavior of PyMongo are correct, the unit tests should be sufficient. But as seen in the past, mistakes happen.

After performing an atomic update, the integration tests reload the document from the database and check it. These new checks also check the internal document . They serve to make sure all of them moving parts come together correctly.