fix base64 code in nbformat.v2 base64 encoding functions were called, but had no effect, because the notebook already has everything as b64-encoded bytestrings, which are valid ascii literals on Python 2. However, the encode/decode logic is actually triggered on Python 3, revealing its errors. This fixes the base64 functions that had no effect to have their intended effect, but does not use them. Rather, it is assumed that bytes objects are already b64-encoded (and thus ascii-safe), which assumption was already made in Python 2.