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.
Are we sure this is a safe assumption? A compressed binary payload, like a tarball, would be mangled by the UTF8 decoding.