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

...d_mode="new_episodes", vcr.py raises UnhandledHTTPRequestError.
Yeah, I just thought it might be good to try to test that after the new episode is recorded, exactly two calls to the url are accepted i.e. the recording under new_episodes has the intended effect. Even better would be a test that makes sure the responses are delivered in the expected order. Not a big deal though. This looks good!