Proposed fix for encoding issue

Possible fix for <a href="https://github.com/robfletcher/betamax/issues/135">Issue #135</a>.

The issue was that binary response bodies were being saved in encoded form, and played back after being encoded once more in BetamaxFilters. The fix was to save compressed response bodies in human readable form on tape and encode on their way to the browser.
This only decompresses the zipped file, whatever it may have within. If we would like to achieve readability for archives in the recordings, I believe that is a separate issue, no?