Proposed fix for encoding issue

Proposed fix for <a href="https://github.com/robfletcher/betamax/issues/135">Issue #135</a>. Haven't written any tests yet, but am planning too.

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.