Fixes #123. When attempting to replay the same request twice using recor...

...d_mode="new_episodes", vcr.py raises UnhandledHTTPRequestError.
I've added a test to ensure that the cassette created with "new_episodes" has different expected behavior when opened with "once". 

I'm not sure, however, on what you mean about making assertions on the responses themselves. Certainly none of the other tests in the file make assertions about the responses.