Fixes#324.
Fixes#1027.
Instead of converting buffers to strings and back again to char array
directly use the buffer data in hmac_init (same as in hmac_update).
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.