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.
Not exactly: AbstractEncoder#decode(), which we're now calling via decompressZipData(), parses the (decompressed) byte[] as if it were a UTF8 string, using guava CharStreams. If the body is *not* a String (say, a PNG image), this will destroy the data.

I added a test demonstrating this (for POST request bodies) as #148