buffer, crypto: fix buffer decoding

Before this commit, DecodeWrite() mistakenly tried to convert buffers to
UTF-8 strings which:

  a) produced invalid character sequences when the buffer contained
     octets > 127, and
  b) lead to spurious test failures because DecodeWrite() wrote less bytes
     than DecodeBytes() said it would, with the remainder either containing
     zeros or garbage

Fix that by simply copying the buffer's data to the target buffer when the
encoding is BINARY or by converting the buffer to a binary string when it's
UTF8 or ASCII.

Fixes #3651, #3866.