Multiple requests to the same URI/method return results as recorded

Hi Dustin,

First of all, thanks for creating VCRURLConnection!  We've loved using VCR on ruby projects and were excited to find someone maintaining a objective-c version.  We're using it for all of our acceptance tests on our new mobile app.

We noticed that multiple requests to the same URI/method would return the same result.  This pull request is a fix that we're using from our forked version.  We'd love to get your feedback on this approach and ultimately have this functionality contributed back into your version.

We're under the pump at the moment trying to get a release out, ideally we'd like to clean up and test the logic in recordingForRequestKey:key some more.  We thought we'd reach out to you about the general approach first.

Thanks again, we look forward to your response.