Crypto update should only accept strings / buffers

I have seen a lot of people trying to pass objects to crypto's update
functions, assuming that it would somehow serialize the object before
hashing.

In reality, the object was converted to '[object Object]' which was
then hashed, without any error message showing.

This patch modifies the DecodeBytes function (used exclusively by
crypto at this point) to complain when receiving anything but a
string or buffer.

Overall this should be a less-suprising, more robust behavior.